Any faction can have whatever another faction has, merely by coding it into the .xml for it. You'd probably want to rename it just to avoid issues, but it'd be possible to give the "tech" faction magic users. An issue to be aware of is that magic and tech use different resources, so balance carefully (because tech doesn't have an "energy" resource it can be difficult to determine an appropriate cost in gold, stone, and/or wood). You could, of course, GIVE the tech side a magic resource.
A simple example would be to take the summoner's .xml code for the appropriate action (casting a spell, for instance) and pasting it into your modified archer/soldier. It's a little more complex than that, but do-able.