Yes, that's possible. It's not even complicated. It's how you use the Desktop / Rack model with any controller keyboard in a live setting. The synth section of all Virus TI's are identical, so the Polar is Desktop with a built-in keyboard. General steps:
Also very helpful: Read up on the section about Multis in the User Manual 
doesn't Local Off mean that I can't play the keyboard of the Polar anymore? I want to use both keyboards.
Indeed I misread your request. In that case, set Local ON.
However, you now really have to carefully set up the Multi Parts key ranges and switch off unused Parts or you will have some unexlected results.
It seems that the Polar plays the MIDI channel of the currently selected Part. The currently selected Part is stored with the Multi, so if you change Multis, the selected Part will change accordingly.
Say you want to play the sounds 1, 2 and 3 with the external keyboard (which sends on MIDI channel 2) and sound 4 with the Polar keyboard:
Now your Polar's keyboard will control Part 04, while your external keyboard controls Parts 1, 2 and 3.
Does that help?
PS: I just realized that this means you could also "abuse" the Multis to be not only one set (e.g. for a song) but also to have several sets/groups within the Multi by assigning several Parts to the same channel and then switching between the selected Parts:
This way you can build keyboard setups for sections of a song and access them by just selecting a different Part.
