Q:

How close can soldered and threaded joints be? 

I would like to extend this pipe (outdoor sillcock) with copper by soldering a female adapter on the extending pipe and then putting this threaded male into it.  Then, I would like to continue on from the other end of the extending pipe (roughly 6") by using elbows etc, which would involve soldering (high heat) near a threaded joint that has pipe dope or teflon tape and my concern is, considering that copper conducts heat very well, whether that's too much heat in close proximity (a few inches) to the thread sealant, which may severely compromise it. 

A:

If I cannot unscrew the pipe and solder it I will wrap a very wet rag to the end that has the combination plastic/copper fittings and sweat the new fittings on.  I use Mapp Gas as it is hotter and works better with non lead solders than propane.  I do burnish the pipe and fitting with abrasive and use water based flux.  I have done this as close as 3" to a composite fitting without damage.  If you notice the rag starting to steam you may need more water on it “dripping wet” keeps the heat from traveling and melting the plastic parts.