New credit after the preliminary combined review process

You mean after you received the preliminary review comment? I don't see a problem in that. However the problem is where do you find an extra credit at this time since it is a combined review. If it is a split review, then there is a flexibility to some modification to the design. However may be it is still possible to target an innovation credit (Ex. Education and awareness program option). Anyway if there is possibility of adding a new credit, in principle i don't see a problem with that.

I've had situations where the documentation for a particular credit simply wasn't available until after the preliminary review. Of course the main problem with submitting the credit, for the first time in the final review, is you only have one chance to get it right.

You can always attempt a new credit in the Combined Final Application. The only thing to be aware of is that the credit will only receive one round of review (as that Combined Final Review is the last regular review phase). You'll want to make sure that your documentation is really clear and that you've provided everything required. If anything should come up during that review, you would need to appeal the credit in order to provide clarifications (as there simply aren't any more regular review phases after that).