Hello Cathy and Guillaume,
Given that the AIS in the B&G is only a receiver not a transponder, you should have no trouble with a splitter between the VHF and the antenna to feed an external AIS transponder.
Certainly making use of the VHF antenna on top of the main mast gives you the ultimate in terms of distance.
We have a Furuno VHF, a GME splitter and a an AMEC AIS transponder and it all works seamlessly well.
You could also ask B&G.