I believe there is someone out there that makes a combo. I had the fox chest protector first so I just bought a camelbak. I wear it over the chest protector. It has worked great for me for the last couple of years riding the dunes and at several XC races.
I use a Fly Racing convertible chest protector/ hydration pack.
JT Cycle sells them on fleaBay check out Item number: 280206102425.
Mine works great, fits well, & doesn't bounce around or bulk up too bad.