This is not a trivial issue. The second part is certainly easy - once you get the Domain B info into Domain A, just wipe the Domain B master and start over to build the Domain A media server. The first part - getting the B info into A - is NOT easy.
However, merging the catalog and the media information is not something you can easily do yourself unless you're an expert. The simplest, and hopefully it will work for you because you said B is small, is to simply import all of B's tapes into the A master. You then have a fully supported solution. Once the import is done, you can do your v7 upgrade and then build your new media server.