Tvheadend

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?

Unfortunately the HURO xmltv grabber grabs only a one liner for each epg entry, while the OTA gets nice descriptions. The OTA does not have the episode numbering of the TV shows (e.g.: 3x12 or s03e12...), which is available in the xmltv output...

Well then if I understand the problem correctly, you would need some tool to extract the data already in the TVHeadEnd EPG database to XMLTV format, then merge in the data you want, then re-import it. Or alternately a tool to read what's already in the database and then add in your data in the appropriate fields. Unfortunately, AFAIK no such tools exist, but if I'm wrong maybe someone else will chime in and give you some better advice.

I actually have no idea how TVHeadEnd stores its EPG data internally, so even if someone could tell you that it might give you a start.