Thats what everyone is doing now ^^
its basically like this, all stages have multiple Model Datas its what makes up the stage
if you have the same amount of model datas as the stage you want to replace (without .rels) then you can replace it, sounds complicated but here's an example
-Online Training's .pac - -Final Destination's .pac-
Model Data[1] Model Data[1]
Model Data[2]
to make an Online Training based stage work on Final Destination, you just have to port a Model Data (usually should just be the static Model Data Mewtwo Created) and make it a Model Data [2] this can work on massive scales such as having an online training based stage work on Battlefield by porting Model Datas[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 200]
If im unclear you can kinda go
Here to view MewTwo 2000s tutorial on Model Datas, it can work for most stages