>I'm using VFP 5 (ancient I know) and it doesn't seem to have this command.

It was introduced in VFP6SP3, I think. In VFP5 you can use LLFF. Something like (not tested).

lcFileName = < ... >
IF FILE(lcFileName)
  lnFH = FOPEN(lcFileName, 2)
  IF lnFH < 1
    * Process error
  ENDIF
  * Move pointer to the end of the file
  = FSEEK(lnFH, 0, 2)
ELSE
  lnFH = FCREATE(lcFileName)
  IF lnFH < 1
    * Process error
  ENDIF
ENDIF