Yes it should be possible to do with minimal effort. I was looking at doing it myself, but I dont have enough experience with Inform6 and could use some help creating a simple test environment to implement Inform6 support in.
In theory the way Inform6 support would work is that the compiler(s) would be added to the ~/.vimform7/Compilers folder. Then the vimform7-port-prj.sh script would be modified with a new I6 parameter and to dynamically generate a makefile with the appropriate I6 compiler commands. If a help manual in HTML format exists for I6 it could also be included in the same way that the I7 help manual is included. I was also considering adding ZIL support to it in a similar fashion.
If someone can point me at a simple I6 “helloworld” with instructions on where to obtain and how to use the I6 compiler from the commandline and maybe a simple IF work that includes use of an extension, I would be happy to add the features to vimform7.
In my experience implementing I7 functionality I noticed the I6 compiler was used for an intermediate build step. If compiling I6 code is the same as that, then some of the functionality already exists.