There is no simple solution, but there are lot of extremely powerful solutions. Here are some ways to
try though:
1) You could learn how to
make maps. Maps can include arbitrary code in them, entirely redefining the rules of the game if you need that (new units, new mechanics, new models, everything). Mapping for Spring is not easy, though: you'll have to learn at least a bit of Lua programming (for your mission triggers; you can get through actual mapping mostly without it) and quite probably a lot more (texturing, for example). It's possible to reuse existing maps though when making a "scripted" version of them of course.
2) You could try how to make scenarios in SpringBoard. After all, initially it was supposed to be a _scenario editor_. It's a bit rough on the edges though, and hasn't been kept very up to date. Something exported from SB is likely to be more or less a mod-map like you would get in 1), but perhaps in an easier way. The bonus point is that you possibly won't have to learn programming.
3) You could try to switch your install into
developer mode and attempt to configure your own campaign with the lobby's campaign framework.
IIRC the lobby even comes with a way to switch the single-player stuff into things other than the default campaign. This is entirely undocumented afaik, except for the
examples that the existing code represents, and the possibility to extract more information from
Aquanim or
GoogleFrog who created this system.
For this approach, you would almost inevitably have to learn programming and other stuff, but as a bonus, you can probably make not just a mission, but a whole campaign.