In the end, it's a quest of trust. Hire people who work on the open source projects your company depends on. Then, let them keep doing it--for pay...at last!--and expect them to continue to balance the worlds of open source and business...as they likely already were, but now with the needle tilted slightly (or more, one would hope) in favor of working on open source code.
It's like picking flowers. If you pick them and bring them inside, they die. Plant them in some fertile ground, however, and let them keep doing their thing. (OK, that was a bit strained...but hopefully you see my point ^_^).