Indeed... one can use the ?path variable to return the path the macro is in. Provided the merge files are in the same path, something like this would work as a standalone macro:
MergeRun(FormFileType: FormFile!; FormFile: ?Path+"date.frm"; DataFileType: DataFile!; DataFile: ?Path+"date.dat"; OutputFileType: ToCurrentDoc!)
NOTE: This will fail if one puts this into an EMBEDMACRO() merge command as WordPerfect writes the content of the macro to a temporary file, compiles it, then runs it.