Use consultants.

(Please excuse me for being glib...)

Seriously though. There has been some work on MEP for ArchiCAD, but from what I understand it either didn't work well enough (ie didn't scale up to the big buildings) or perhaps they just didn't have the where with all to compete with the established players.

For doing basic mechanical stuff like exposed ducts and such there are probably some library parts available and Archiforma from Cigraph might do the trick. You can also do a lot with the wall, beam and column tools and custom profiles.

For the basic electrical plans (switches and lighting, power and signal) that architects do there are symbols in the standard library as well as third party parts (like Electrigon).

For plumbing the answer is pretty much the same as ductwork, but since very little plumbing is ever exposed I haven't seen much call for it.