Hârn was published a few years before HârnMaster, and I'm sure quite a good number of players used RuneQuest, StormBringer or Magic World (WoW version) in Hârn before that.
Concerning your "Convocations", I know nothing of Hârn magic, but the table reminds me of Mythras Sorcery. In this game, each sorcerer belongs to a tradition, which teaches a specific Invoke skill, and a listt of spells that can be learned and cast with that Invoke skill. In BRUGE, you could use the Magic rules as a basis, but reduce the number of skills to 6, one per convocation.