can you make royal icing using store bought icing and confectioners sugar? i'm allergic to egg so therefore can't use meringue or egg whit

http://sweetopia.net/2012/10/can-you-use-anything-besides-egg-whites-or-meringue-powder-in-royal-icing/

In this thread there are a couple options towards the bottom of the page:
http://cakecentral.com/t/659310/egg-free-royal-icing