Switch the whole Apex rear derailleur over, or buy the Force medium cage derailleur. Or buy a Rival med cage RD. Or an appropriate Sram mountain RD.

I can't imagine a scenario in which building a Frankenstein RD would be preferable to any of these options. If you want to stay Force for the bling factor, you could get at least very close to even money on swapping the RD out. Heck, you might even be able to make a few bucks on the swap if you're lucky.