When Teclis of Ulthuan and Magnus the Pious founded the Colleges of Magic, they didn't do so from scratch. Using every magical and mundane means at their disposal - and between the High Loremaster of Hoeth and the Emperor, that's a lot of means - they spread word to every corner of the Old World that all magic-users were offered amnesty and training. And when they did so, all sorts of strange sorts came out of the woodwork. The Druids were the most monolithic, to the extent that the Jade Order as a whole could be argued as a mere rebranding rather than a new organization, but by no means the only example. The University of Altdorf was decimated (in the literal rather than figurative sense) when they lost large portions of their Departments of Philosophy, Astronomy, and Chemistry. The 'Guilde of Alkemie' existed for at least a thousand years before the Golden Order was founded, which poached their most skilled members and then started giving rigorous oversight to ensure that their methods remained entirely in the realm of the mundane. Apart from the Hedgewise who semi-openly practiced magic, a good number of wandering soothsayers, tinkers, herbalists and circusfolk came forward, revealing the hidden magical underpinnings of master-and-apprentice traditions centuries or millennia old. And even though Teclis did not challenge the Empire's religious orders when they said that Divine and Arcane were completely unrelated, more than a few priests decided they were more attracted to the trappings of their deity than the deity proper and joined the Colleges instead.
The founding of the Colleges did not introduce magic to the Empire. It transformed what already existed from the equivalent of cottage industries to the efficient and regimented campuses of Altdorf.
