Yes, it can be upgraded, by Maheer.
In Waukeens Promenade, near the painter, in the west raised section, there is someone called "Shopkeep". When you talk to him however, he refers to himself as "Maheer". If you bring him the horn, and question him about magical weapons he makes, it can be upgraded for......
1st Upgrade: 2000gp, Diamond, Normal Horn
2nd Upgrade: 5000gp, Beljuril Gem, Once Upgraded Horn
To get a beljuril stone, either.......
1) Go to Umar Hills and hack all the chickens apart
2) Talk to a guy due North-West of the Cowled Wizards house. He'll tell you he knows a good adventure. Pay his price, and he'll tell you a story. Go to the chicken herder and talk to him, question him about the gem, and he'll give it to you.
Just post or PM me if you need some more help.
DJG