Would it be possible to take a bsp from halo2's campaign and put it into custom edition to use in my own single player map? (I could make do with a section from Combat Evolved's campaign but the courtyard in the beginning outskirts would be absolutely perfect)

well you can. Three options here:

1. Rip it from halo 2 directly using certain programs. Be warned though, this would require you to fix many bsp errors before you can import the bsp into halo.

2. Ask from someone on this forum (not me, because i dont have it), maybe the older members. Some of them do have sections of Halo 2 bsps (hopefully fixed), or may be willing to rip/model the required sections of H2 bsp for you.

3. Get the H2 Turf bsp from here
http://hce.halomaps.org/index.cfm?fid=5205
its a good alternative to the outskirts courtyard, albeit kinda small.

1 and 2 I can try but I specifically wanted the courtyard because of it's layout of buildings and pathways, not just because it's a human city.