How do you flavor Italian Buttercream?

Another way to add your nut and fruit flavors to a cake if you are having trouble incorporating them into the buttercream is to put them between layers or add as a moistening liquid.
Then ice with plain buttercream.

I've used pretty much any liquid to flavor buttercream but my usual method is to pull out a small amount of the buttercream and whisk liquid into it (like concentrated instant coffee) before incorporating it back in. With fruit flavorings, however, its tough to pack a lot of fruit flavor into the buttercream without watering it down. I would recommend a fruit puree (either bought or made from fresh/frozen fruit) or a freeze-dried fruit powder. The freeze-dried powder sounds artificial, but its actually going to give you the most flavor, without adding liquid to your buttercream. Another trick I use is adding a little citric acid (you can get it at some specialty spice shops and online) to the buttercream to give it a slight zing. You'd be surprised how much flavor it adds when you're using a very sweet, somewhat dull, fruit puree. I would, for the most part, steer clear or cooked fruit, reduced fruit juice or preserves. However, I've had some delicious results with blackberry, boysenberry and apricot preserves.

I've never had problems adding nut pastes to buttercream but I pretty much use Fabbri exclusively, so I'm not entirely sure how homemade nut paste behaves. I don't know if you're familiar with praline pastes, but they can be quite useful in buttercreams. Just make a caramel (just a caramel, not caramel sauce), stir in the nuts and let cool. Then break it up and process it until a paste. I've had good results with that.