ok, each .map file has not only the ****.map name.. but also has a certain files size and internal name that the XBE reads.. those SP maps that can be played in MP arn't the actually sp maps at all, they are whats called conversions. That's acheived by ( 1 meathod ) taking the SP maps BSP and editing it with 3dsmax or somthing and converted to .obj. and injected/replaces the multiplayer bsp that you choose. so then you have to set all the spawns, flags, mp flags, all that plus collisions, dependancies ( sp? ) and so on.. ANYWAY.. so what your going to have to do is this -
Ok so say you want a map called " XXX " replaced with a map " ZZZ "
you'll have to find what the internal name of xxx is, and it's files size, and it's physical file name.
and switch zzz to those settings for it to work. plus to make it right, you should also add the correct description and dxt pic. thats done through the UI.map.. If you really want to do this and you havn't understood what I have said so far, check some other places like halomaps.com or halomods.com
if you need further help, I can still help.. buttom line is though that like 343guilty said.. your going to probably have to hack the XBE for it to work properly.. because you can make a small map size big, but not a big map size small.. to do this without alot of help your going to need some type of halo modding backround.. it's not hard, but you need to know what your doing
note - I do this all the time because somtimes I don't like peoples packs so i add them together and what not.. and plus I also make my own packs