I'm honestly not sure, I have no experience with Objective-C but just from reading I think you'll still need to write an Objective-C interface/wrapper for your C code before it can be interfaced with RubyMotion.

Edit: A discussion topic on the newsgroup about it: https://groups.google.com/d/msg/rubymotion/tn5p3g071OE/5jyJI... (you'll have to sign up I think)