Welcome, assailant.
Yes, you can run any combination of plugins and scripts using a batch file. The structure of batch files is simple, for example...
NAME=IMDB+Allmovie
DESC=Get movie information from IMDB.com and AllMovie.com
TYPE=2
EXECUTE=imdb.dll|allmovie.psf
...is one of the default ones provided. Put your batch file in the Plugins directory, and it will appear in the Import menu the next time the program is started.
When using more than one source, you have to configure which has priority on a field level. This is done using the Overwrite fields... setting for each plugin/script in Preferences. Each field can be set to be overwritten, added only if there's nothing there already, or ignored.
I'm not 100% sure how this works, but I think the way to get the result you want is to de-select all fields for the IMDb plugin—so it will only get IMDb rating. Select all fields for OFDb—including Additional rating.