Okay, well first, no you cannot add maps in addition to the original maps that came with halo, they must be overwritten. Second, there haven't been any new maps made with a completely new BSP (the actually level like the ground and bases etc.), but there have been mades made with building blocks such as Galite and SkyTore, which are part of the NMP mod. Third, Halo came out prior to the introduction of Xbox Live so it is not compatible with it at all. Halo lacks the necessary "hooks" to be updated online or through an expansion disk.
However, xorange's Halo:CE to Halo Xbox tutorial is beginning to show some hope of new content on the Xbox version, after sucessfully importing a perfect model he made in gmax with a perfect texture and collision model so maybe this summer we could see some cool stuff.