Welcome, handytouch.
It's not currently possible. The plugin would have to be modified, including providing for the configuration of a custom field to receive the data (as has been done to date for Votes, Rank, Technical Specs and User Review). But there's a lot of information in IMDb that's not downloaded. For something like this to be considered, I think it needs to have merit by itself, rather than as a supplement to another field.
There's no MPAA rating for about 5% of the movies in my database. Most of those are old, foreign or obscure films which have no certifications that mean much to me. Some have no certifications at all. So I don't think having the certification data as well as the MPAA rating would help in my circumstances. If I cared, I'd deal with these exceptions by reviewing the existing certifications (an possibly other sources), and then manually entering what I believe is the equivalent MPAA rating. That way, I would have one field with a rating for all movies, instead of a second field that is 95% redundant and the other 5% of questionable value.
Maybe what would be more useful would be for the Certification data to be download to the MPAA field in cases where no MPAA rating exists.
