Relative newbie here. I've been baking since I was a kid but I generally full cakes with the same frosting I use on the outside.

I'm making my daughter's cake this week. It will be (beyond buttercream) yellow cake with (SMBC) chocolate buttercream frosting and a raspberry filling.

I have a jar of really good raspberry jam to start with but I'm wondering how/if I should doctor it.

Plan A is to just put it in the mixer to loosen it up but I'm wondering if I should add anything to it? Or beat it into vanilla buttercream?

I'm sure either would be fine but I'd still love opinions.

If it's really good jam I'd torte the cake and do a layer of chocolate in the very middle, then two thin layers of the jam on the top and bottom sections. I wouldn't put it into confectioner's sugar buttercream if I was going to add it to icing, but if you put it into a meringue buttercream that would work.

Why not put it in the middle, too?

I'm going to follow your advice, I just want to know why. :)

Well, if you like the jam go for it and use it as a filling. Or mix it with your smbc to have a bit more creaminess inside your cake. Or make a raspberry puree reduction which I find is less artificial and sweet than jam...A bit more zingy I would say... and use it as a filling or mix with smbc (or whatever bc you use). 

 One of my favorite fillings.  I sometimes fold in about 1/4 cup of raspberry jam into the cake batter also.  For the filling I generally use a layer of white chocolate ganache then a layer of raspberry jam (the seedless).  Sometimes I also use a layer of SMBC then a layer of raspberry jam.  Either way, it taste great.

You should really check this out:

she uses a type of blackberry but I'll bet raspberry would be amazing as well!