Assuming I can't use my PBDs before the end of the year, but I can before the end of March, it sounds like the days would first have to be dropped as a Personals drop which has huge limits on my category due to the way they staff reserves. They'd then need to be converted to PBDs, is this correct? Lately it seems like the chances of a personal drop are slim to none, and the APD has been used already.

If used as supp vacation do they need to go through the standard vacation bid?

You may find in the winter you can easily drop trips. If not the trip will stay in open time as a qualified trip and a couple of your trips will likely get picked up and become a drop for you.
Another option would be to use your APD.

You can use your PB days from this year in Jan-Mar 2017. You will have already bid (or be in the process of bidding) them as supplemental vacation days. This is why you will need to call scheduling after you drop a trip. Also be aware that Jan-Mar of any year is the ONLY time you can use PB days to cover an APD.

Denny