I think I need to get a bit more specific with my question. I'm doing a monstrous character, and for spoilery reasons it would be most convenient to do so via "monster class levels" of the kind found in Savage Species, Libris Mortis, and the savage progressions from the online enhancements (maybe a few other places, too). However, the viability of this drops significantly if every one of these levels adds +1 to the total CR. Can I just add the CR of the original monster instead?