I think, that your files contains not many lines.
Read the Const.txt file into the user variable named const (see task below) like comma separated text. Next you can access to each line as const(i) where i is line number.
And main problem, that this script must started when I click on "NEW Message" button in Outlook, and result text from WRITE.TXT must append as signature to message.
You can form the write.txt file and next force Outlook Express to begin a new message.
For example see task below. In order to import this task into RoboTask save text of task to any *.tsk file and use "Task | Import " menu command.
;**********************
;* RoboTask Task file *
;* Do not edit! *
;**********************
[Root]
ActionAfterRun=INTEGER|0
Actions=FOLDER
Automat=INTEGER|-1
CatID=INTEGER|565106205
ContinueOnError=INTEGER|0
ExternalName=STRING|"Task185"
Hide=INTEGER|0
ID=INTEGER|1792563564
Name=STRING|"Combine two texts demo"
Priority=INTEGER|3
RunOnClose=INTEGER|0
RunOnStartup=INTEGER|0
ToLog=INTEGER|3
[Actions]
Action1=FOLDER
Action10=FOLDER
Action11=FOLDER
Action12=FOLDER
Action13=FOLDER
Action14=FOLDER
Action15=FOLDER
Action16=FOLDER
Action17=FOLDER
Action2=FOLDER
Action3=FOLDER
Action4=FOLDER
Action5=FOLDER
Action6=FOLDER
Action7=FOLDER
Action8=FOLDER
Action9=FOLDER
[Actions\Action1]
ActionID=STRING|"A_FILE_DELETE"
Enabled=INTEGER|-1
Name=STRING|"Delete File"
Params=FOLDER
[Actions\Action1\Params]
Count=STRING|"1"
deletedirs=STRING|"0"
file0=STRING|"c:\write.txt"
subdir=STRING|"0"
ToRecycleBin=STRING|"0"
[Actions\Action10]
ActionID=STRING|"A_VARIABLES_SET"
Enabled=INTEGER|-1
Name=STRING|"Set variable ""LINE_NO"" with value ""0"""
Params=FOLDER
[Actions\Action10\Params]
expand=STRING|"0"
varname=STRING|"LINE_NO"
varvalue=STRING|"0"
[Actions\Action11]
ActionID=STRING|"A_LOOP_TEXT"
Enabled=INTEGER|-1
Name=STRING|"Text Loop"
Params=FOLDER
[Actions\Action11\Params]
destvar=STRING|"LINE"
file=STRING|"c:\text.txt"
linecount=STRING|"0"
sourcetext=STRING|"0"
[Actions\Action12]
ActionID=STRING|"A_FILE_TEXTWRITE"
Enabled=INTEGER|-1
Name=STRING|"Create text file c:\write.txt"
Params=FOLDER
[Actions\Action12\Params]
fileexists=STRING|"1"
filname=STRING|"c:\write.txt"
line0=STRING|"{line}"
line1=STRING|"{Const({Line_No})}"
linecount=STRING|"2"
[Actions\Action13]
ActionID=STRING|"A_LOOP_END"
Enabled=INTEGER|-1
Name=STRING|"End Loop"
[Actions\Action14]
ActionID=STRING|"A_GENERAL_SENDKEYS"
Enabled=INTEGER|-1
Name=STRING|"Send keys to ""- Outlook Express"""
Params=FOLDER
[Actions\Action14\Params]
currentwindow=STRING|"0"
fixedwindow=STRING|"0"
keylayout=STRING|"0"
keys=STRING|"""%(F){ENTER}{ENTER}{DELAY 300}{TAB}{TAB}{TAB}"""
sendkind=STRING|"0"
wincaption=STRING|"- Outlook Express"
[Actions\Action15]
ActionID=STRING|"A_GENERAL_SENDKEYS"
Enabled=INTEGER|-1
Name=STRING|"Send keys to active window"
Params=FOLDER
[Actions\Action15\Params]
currentwindow=STRING|"1"
fixedwindow=STRING|"0"
keylayout=STRING|"0"
keys=STRING|"""%(I){DOWN}{ENTER}{DELAY 100}c:\write.txt{enter}"""
sendkind=STRING|"0"
wincaption=STRING|"- Outlook Express"
[Actions\Action16]
ActionID=STRING|"A_VARIABLES_REMOVE"
Enabled=INTEGER|-1
Name=STRING|"Remove variable ""LINE"""
Params=FOLDER
[Actions\Action16\Params]
varname=STRING|"LINE"
[Actions\Action17]
ActionID=STRING|"A_VARIABLES_REMOVE"
Enabled=INTEGER|-1
Name=STRING|"Remove variable ""CONST"""
Params=FOLDER
[Actions\Action17\Params]
varname=STRING|"CONST"
[Actions\Action2]
ActionID=STRING|"A_VARIABLES_SET"
Enabled=INTEGER|-1
Name=STRING|"Set variable ""CONST"" with value """""
Params=FOLDER
[Actions\Action2\Params]
expand=STRING|"0"
varname=STRING|"CONST"
[Actions\Action3]
ActionID=STRING|"A_LOOP_TEXT"
Enabled=INTEGER|-1
Name=STRING|"Text Loop"
Params=FOLDER
[Actions\Action3\Params]
destvar=STRING|"LINE"
file=STRING|"c:\const.txt"
linecount=STRING|"0"
sourcetext=STRING|"0"
[Actions\Action4]
ActionID=STRING|"A_FLOW_IF"
Enabled=INTEGER|-1
Name=STRING|"If Then"
Params=FOLDER
[Actions\Action4\Params]
case=STRING|"0"
operator=STRING|"7"
type=STRING|"0"
value1=STRING|"{const}"
[Actions\Action5]
ActionID=STRING|"A_VARIABLES_SET"
Enabled=INTEGER|-1
Name=STRING|"Set variable ""CONST"" with value """"{line}"""""
Params=FOLDER
[Actions\Action5\Params]
expand=STRING|"1"
varname=STRING|"CONST"
varvalue=STRING|"""{line}"""
[Actions\Action6]
ActionID=STRING|"A_FLOW_ELSE"
Enabled=INTEGER|-1
Name=STRING|"Else"
[Actions\Action7]
ActionID=STRING|"A_VARIABLES_SET"
Enabled=INTEGER|-1
Name=STRING|"Set variable ""CONST"" with value ""{const},""{line}"""""
Params=FOLDER
[Actions\Action7\Params]
expand=STRING|"1"
varname=STRING|"CONST"
varvalue=STRING|"{const},""{line}"""
[Actions\Action8]
ActionID=STRING|"A_FLOW_ENDIF"
Enabled=INTEGER|-1
Name=STRING|"End If"
[Actions\Action9]
ActionID=STRING|"A_LOOP_END"
Enabled=INTEGER|-1
Name=STRING|"End Loop"
;end of task