I see it doable in 2 ways... (posibly not as house rules either)
First; They use there Technical(Mechanical) Skill to create the nano-machines, giving them either the ability to self replicate, or simply building a hug mass of them. The requirements to make the building role would require both high ranks, as well as rolls in the fallowing skills; Knowledge: Alien Life, Knowledge: Nano-tech, Profession; Robotic Engineer, Medical, and maybe more. Also a considerable amount of time may also be required. They may require what ever is needed for masterwork idems as well... Doing it this way may be dependent on the actual level of technology and resources availible at the time. (as the shadows direct minions, the Drak probibly gave them a lot of support at verious times) This could be the methode that takes the more time, maybe years of study, testing, and development.
Second way: Using the idem creation powers, you use a spell (may need to be created, or appears in future books) that is able to aflict disease, biological harm on a target. Add in the Spell for to make it micro-sized, and self generating power. Make a hundred, or much more... then invect some one. They may spead them to others seeing as how the effect may be sickness... The viris will go dormant from time to time to recharge, then hit again. Also with other spells, its at complete control of the Techno-mage.... [In Crusade, one episode, a technomage, had build a machine that controled (posibly produced) nano-machines on an entire planet. this could be operated by a holodemon] There are other creation spells that could make it work, and maybe more that could be created/or will show up in future books. (Self-Replicating idem, or others)
when i can look at the book after work i can make more accurate judgements but i think its pretty good off my head here