Seamus O'Toole was a dwarf sorcerer in "Blowing Sky High" in the Shadowrun Returns Anthology. By the time we see him again in Sail Away, Sweet Sister, he's a vampire with a fire elemental in his front yard. He had to get that from someplace, and a sorcerer couldn't have summoned him....
I don't think I blatantly spelled it out, but his Infection coaxed the rest of his Talent out of him; he's a full-blown mage now, though his conjuring and such definitely take a back seat to his sorcery. I'm pretty sure I left enough crumbs to follow that trail, though.