The 3800 is capable of Ip trunking in conjunction with the other hipath systems, as well as SIP. To integrate that into a Panasonic would be questionable.
Any IP connectivity will have to be achieved with a HG1500 card for each system and it has to be licensed accordingly from V5.0 upwards. Needless to say that the Panasonic will in theory be able to connect and the link be established.
The HG1500 could be programmed to point the relevant traffic to the Panasonic node and so on, i would be sceptical of the chances of it working and then again why would you go down a route that would be unknown and with a likelyhood of failure from the start.
The main problem with anything like this is that each vendor Siemens/Panasonic will not support any integration between the two.
Qsig.
you are not going to be able, even if you connect to the Panasonic system with Qsig (not actual Qsig but Hipath Hybid) to have any feature transparency.
I have in the past set up Qsig connections between Mitel and Siemens Hipath, and ISDX and Siemens Hipath. Needless to say if you want to be able to make a phone ring and transfer a call then this is a good option.
My advice would be to install a Siemens Hipath and Hg1500 card, in the new site, you know it is going to work and be easily integrated into the current network.
Alternatively why not just install IP phones from the 3800 in this site? License the 3800 with the relevant license and then remotely connect the two.
There would be no need for any system at the new site. Also this would be possible with just a VPN connection, dependable on the bandwidth.
The real question is why would you install a Panasonic at all into an existing Hipath network?
If you are looking at integration issues such as outlook and the like then the Siemens Hipath does this also, for existing sites a server can be used to provide these services.
Again the new site could look at the Openscape MX/LX product which can be introduced into an current hipath network.
Unless you were looking to change all of them out at a future date then it does not make sense?
If you are having difficulties within the existing set up, or need further technical assistance then please feel free to contact me.