have you installed game and it's expantion in dosbox? if not, do so.

yes, I installed the expansion via dosbox, but the program crashes after install (DOSBox quits)

have you installed main game in dosbox too?

yes, the main game works though.

Something similar happened to me and I also installed first TIE Fighter, then Defender of The Empire expansion from original floppies WITHIN DOSBox, though DOSBox didn't quit, it just got frozen. However, I *KNOW* why did it happen so if your problem comes to be the same I'll try to help you there.

See, my problem was related to the file FRONT.OVL which is the file which needs to be patched to bypass the copy protection at the start of the game. It's not that I don't have the manual which comes with the game including copy protection codes along its pages, it's just that I always found it quite annoying so I just bypassed it modifying this file.

But behold! Know that DoE installs its OWN FRONT.OVL which is different than the original one and it only does so if the copy it finds BEFORE installing is the original one. So you need the UNPATCHED FRONT.OVL there before installing DoE, then patch (or not, your choice...) DoE's exclusive FRONT.OVL for the game to work. But be careful, the patching IS NOT the same for this new version. I'd tell you where to find both patches/hex code modifications but I'm not sure if it would qualify as low level piracy around here and it's not too hard to find them on the net if you look carefully.

Hope to have been of some help for you! 😉