Is it possible to put the premade scenario families into a save we already have to play?

If you mean after you finish the scenario, you just save them to your Library and then put them in the save you want them to be a part of.

If you mean before starting the scenario, make sure the save you want to play them in is the most recent save. You can do that by loading the save and just saving (or Save as if you don’t want to over write your current save). Then you just move them in as normal.

I hope that makes sense. If not I'll gladly clarify anything you need me to. :)