The SET TEXTMERGE command is perfect for adding to an existing text file. Have a play with this code.

*-- Pick the file to add text to
SET TEXTMERGE TO "C:\MY TEXT FILE.TXT" ADDITIVE

*-- Turn text merge on
SET TEXTMERGE ON NOSHOW

*-- Add the text to the file
\\this text will be added to the end of "my text file.txt"
\so will this
\also you can put VFP variables in <> <>
\as long as they are enclosed in cheverons
\the single slash and double slash have different meanings
\so look them up in the help.
\Don't forget the ADDITIVE clause on the SET TEXTMERGE TO

*-- Turn text merge off
SET TEXTMERGE OFF
SET TEXTMERGE TO

*-- View output
MODI COMM "C:\MY TEXT FILE.TXT"