Meringue Powder will stiffen it. Also, Powdered Sugar is a good stiffener, just add a little at a time until it's the consistency you want. 
Good luck! 
I've used Meringue Powder and Powdered sugar also. They both work great! I usually add a bit more flavoring if I have to use a lot of Powdered Sugar, about a quarter tsp at a time. That should get you what you're looking for! One time I thought I had all the ingredients for a chocolate buttercream but i was wrong...... Didnt have time to go to the store so I used a jar of store-bought chocolate frosting and added a pkg of instant chocolate pudding.... Not exactly the taste i was going for but the consistency was pretty good and it piped really well! Just an idea! I have only done it 2 or 3 times since when I'm in a pinch... No one can tell it wasnt from scratch!
good luck!
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