add it in MMB afterwards?
Welcome.
I haven't done this with hotels, but have done with cars and can say for sure the math works.
If you book package, you don't benefit from the option of hand-baggage-only fares... so can't do HBO+car. I've test-booked this a few different ways and the end result price comes out same: book the airfare, then go to MMB and add the car.
Caveat that the car bit is cheaper and probably simpler for the pricing computers than the hotels... so I'd be interested to hear if you got the same result, if you try it. I was happy enough to experiment since the difference was only ten or twenty either way.