am not sure about changing the existing volumes/partitions on the fly to get a 3rd ntfs one, but if you just want to have something in nfts one way could be to use a container file, but to simply store it as a regular file on the regular drobo volume.
for example i have one on an existing drobo volume, which has the cddb database. its very big about 10GB and there are probably lots of posts about it on this forum, but i did it essentially, to ensure that i was always able to access the database even if no or slow internet. (and it was better than trying to store millions of little text files on the regular drobo volume)
another idea that might work for an actual separate windows system could be via an oracle virtual box. that way as long as the operating system you are using, can launch virtual box, you could specify the regular file that contains the ntfs virtual windows system.