This question is year specific. If you just loop the tstat housing back to the water pump one on a closed system, you will have major problems. Open one, sure.
There's a nipple-nipple adapter out there that will work fine. They make them to do lots of weird things. Just hit up a decent hardware store, or sometimes auto-parts stores have 'em.
And to drain the heater core, you have a couple of options. With the hoses off it, flip the jeep onto the passenger side. If this doesn't sound ideal, pull the HVAC housing apart (it's held together with a bunch of screws), and pull the core out. At which point I'd recommend throwing a new one in, but that's up to you...