I don't think this is built in to RoboHelp - I don't have the
latest version, though. I take it this file is not HTML. What type
of file might make a difference in how any workaround is
implemeted.
My first thought would be to make it always external but then
it has to be shipped with the CHM file. To link to an external
file, Micosoft has a javascript function called parser which will
find the correct path if it is installed in the same directory as
the .chm. This is explained in HelpWorkshop's help file. I could
send you the code if you'd like.
In using this, I have found that every file type responds
differently - some open directly in help viewer, some open their
own application, and some throw up a dialog that it might be
unsafe.
Another option would be to have it edited while in your
RoboHelp project directory. i.e. consider that file the "original."
Unfortunately, I think you'd have to share the entire project to
allow someone to do this and that might open another can of worms.
Good luck,
John