Does anyone know if there is a tool which can convert ebooks in the standard Rocket format (for REB1100/GEB1150) to the GEB2150 format? GEBLibrarian can do them one at a time, but I have the Blackmask Rocket Ebook collection, and I was hoping to find some tool that would let me convert a couple of hundred at once.

Well, if it's RB format you can batch convert them. You can extract them to HTML using rbburst (http://rbmake.sourceforge.net/rbburst.html) which you can download from the rbmake project (http://downloads.sourceforge.net/rbmake/w32-rbmake-1.1.zip).

For batch conversion from HTML => IMP there's no tool directly available that I know of. It's not too hard to create one, though. If you do know Python, you can take a look at this code (http://pdfread.svn.sourceforge.net/viewvc/pdfread/trunk/src/output.py?view=markup) for a starting point.