You can require copyrights transferred to you from any contractors, but keep in mind the best professionals aren't simply going to agree to it. You'll have to buy our their rights to their work. It's not the same thing as hiring an employee. Either that, or get new (and not as talented) staff who don't know any better. We were actually discussing this in another thread. For example, I usually tack a $500 buy out fee for anyone who wants the copyright to any articles I write, whereas Julian (pretty sure that's who it was) mentioned $10,000 for his design work copyright buy out rate. In other words, true professionals simply aren't going to hand you their copyrights just because you ask them to.
Jenn