Hi. The usual method to add a cross section library is to put the ace files in MCNP_DATA (or wherever you have your data) and make changes to or replace the xsdir file that indexes them. You can also, at a push, use more than one xsdir with different names.
This said, it's normal to use the same standard cross section library as much as possible with a given MCNP version for trust reasons. Change the library and the results won't completely match, and it's possible a new library could crash the program or produce wrong answers.
In my opinion, trust is the reason to use MCNP.