FYI, I do this in FreePBX with a custom trunk (that points to an extension) like this
Custom trunk = SIP/$OUTNUM$@4999
where 4999 is the extension that SIP2skypre registers to.
I then establish extensions for those users that I must reach by skype. In the dial command on the extension I then use SIP/SkypeUserName@4999, and uplink merrily sends the call to skype.
to clarify each extension serves as a skype destination. I could just as well do it this way in the extensions SIP/SKypeUserName@uplinkIP:5070 but I think that what I defined previously is better because it better accommodates an IP address change.
It is cumbersome, but the best solution I have found for the people I must call on Hype.