I'm tempted to buy PB2019 in the Christmas sale but I also want to continue my 2018 dynasties (both pro and college).

Is it possible to continue my PB2018 career and pull rookies from CB2018 in PB2019?

Yes you can import the PB18 game to PB19 and can still use CB18 rookies in PB19. I would recommend waiting until you have ended the PB18 season to import though - the new tactics are a big change in the game so best to let the AI especially sort things out in the offseason in the new game. Also always a good idea to backup the old league before importing just in case.