What version of software do you have on your OS7400?
I just paired a sip extension with a digital under 4.60b

You can pair SIP extensions with virtual SLT's to get around your ring group issue (it's only unconditional ring groups that you can't add sip extensions into)

Your KWS8000 solution is going to be an expensive option to put the SLT cards in it, plus your going to have to put slt ports into the system as well, otherwise your connecting via sip extensions the same as your current dect