You don't need to mix it with anything, just make a buttercream dam to ensure it doesn't seep out.
I like to spread frosting on the cakes that will come in contact with the filling so that it won't make the cake soggy. Also, don't forget about the dam.
I don't mix the filling with buttercream but I do pipe a buttercream dam around the edges of the cake first then fill with pudding but don't overfill. This is so the filling doesn't seep out of the cake. Also, I carve down the cake juuuust a little before putting in the pudding so the cake sits down more level. Pipe the dam, fill with pudding..then place your cake layer on top. I learned this on this site after having a few bulging cakes. 
I filled my Peanutbutter Cake with Chocolate Pudding and dammed it before adding the pudding. It was great.
Lazy_Susan
Also, cooked puddings tend to hold up better than instant because they are less wet or runny. These cakes must always be refrigerated unless you are using a sleeve commercial filling.
Hugs Squirrelly
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