In multiplayer is possible, but in skirmish the spawn menu is already full. We don't want to add too many panel cause it's will make game unstable.

That is understandable, is there any other way I can do this? I don't think it is reasonable for you to make an entire new version just because one person asked, so is there a way I can do this myself? Like a code, or something?

@Abraham Lincoln
Yes, simply open the mod folder and unpack the mpa.pak file with 7zip, find \resource\map\U-47/change_enemy.mi and edit it with a text editor (I use notepad ++). It takrs some time to understanding the code structure, bit it's not very hard. You need to edit tje support menu section and the relative respwan section.

*note: If you unpack the .pak file, you need to move the .pak to somewhere otherwise the game will load the files in pak file first and ignore the uncompressed file you have edited.

Thank you!