ok, this can be someone what complicated. First, does your expansion unit have a doubler so that disks are written in double density? Or is it without a doubler, which means the disks are single density. Just how you approach this largely depends on that one thing. The doubler will be a small daughterboard plugged into the socket where the disk controller chip would largely be.
If it DOES have a doubler, then it's easy enough to write disks using an old PC. You can use the same software I've mentioned in this article. http://www.classic-computers.org.nz/blo ... -trs80.htm
You'd be best not to use my System 80 images directly however if you are going to use a PC to write the disks. You'd be best to use a TRS-80 Model 1 emulator to make some 40 track, double density disk images (a DOS disk of some type and some blanks) then using Matthew Reed's TRSTools (Google it), copy the programs from my images to these new images. These new 40 track images can then be used by the PC program and a 360k drive to make disks which should work on the System 80. If you don't want to go to this bother, I have a collection of favourites in 40 track, double density disk images. Email me and I'll share with you.
If the expansion unit only read single density, then it's a whole new ballgame. There are some old '90s computers that write single density disks but they seem to be the exception rather than the rule.