The simple answer is, yes it is.
But, you would need to know where to obtain the additional information you want to add, you would need to know how to encode it in xmltv format, and then you would need to either add that data in manually using a plain text editor, or you would need to write a script or program to do it automatically.
So it is definitely not impossible, but depending on the format the additional data is in, it could be a real programming challenge if you don't want to do it manually by editing the XML file in a text editor, which I'm guessing you probably don't.
EDIT: Oh, wait, you said you wanted to enrich the OTA Grabber information. That might not be nearly as easy since I don't know if that ever gets converted to an XML file format in the first place. I suspect it goes right into the TVHeadEnd database, so that would make it even more challenging since you would need to figure out how to modify the data that's already in the database. Why not just skip the OTA data and use the EPG data, if it contains the information you want?
