I have the code for the TIE Advanced, and I have a seperate TIE Advanced unit for land battles (similar to snowspeeder). Is it possible to combine them into one unit? I know it's possible with Hero units like Vader, but can it be done for regular units? Thanks for help in advance (no pun intended).

Yes it's possible, but I don't know how to do it. Check the "Victory SD in ground battles" thread (named something like that, somewhere on the forum). The VSDs work both in space and on ground and are the same unit (I think...).