[PF] Multiclassing with official sheet ?

Hello, I just have a quick question. Using the official sheet for Pathfinder (1st ed), is it possible to multiclass by drag-and-dropping the 2 classes from the compendium ? If yes, how ? (I don't seem to be able to do it) If not, is there any other option besides entering the second class by hand ? Bonus question : How about prestige classes ? Do they work the same as standard multiclassing ? Thanks in advance !

No.  The charactermancer for PF1 is only for initial level 1 build (core rule book) only.  You build whatever you need from there, or from the blank sheet if you aren't starting from the core content covered by the charactermancer.