I don't know of any open source Ada-to-C translator. The only one I knew of at all was SofCheck's, which was reportedly pretty good.
SofCheck has since been bought by AdaCore, and I did a very brief search of the AdaCore website for the translator, and nothing jumped out. You could ask them at sales@adacore.com, if pursuing a commercial solution is a viable option for you. (At least get a price.)
