I doubt you'll be able to get the plug hot enough to solder the tubing into it with the plug screwed into the block. The heat sink effect may not let the solder draw into the joint well enough to make a good joint. But you're welcome to try. I don't think you'll have an issue with sealant, as if you get the plug snugly in the block, taper thread tend to self-seal. Pipe dope and/or teflon tape are more of an assist in getting the fittings tight enough to seal.
