at the moment this is not possible .....
There is a feature request to include a htsp client in tvheadend so you can link them together, but that is still a feature request.
And the IPTV streaming options of tvheadend are not compatible with the IPTV incomming requirements. (and there is also a feature request to change that too)
But what you can look into is the vtuner project
http://code.google.com/p/vtuner/
That will allow you to use a dvb tuner in a remote machine as if it was a local tuner.
Hein