I believe the units are saved in semi-readable files, i.e. you can open them with notepad and once you get past the HTML-ish formatting, it should be possible to figure out how most things work.
You could then find traits from canon dropships and copy paste them to make a new one out of traits you want. E.g. if you want to add a light vehicle bay to a Union, you find a dropship that has light vehicles bays, copy paste the section over and adjust the numbers as needed.
It's awkward and has no rule validity checking (so you'd need a copy of Techmanual to do it by hand), but it should work.