mob spawning in minecraft structure

To sort-of answer your question, No, there's not a great way to do this. But I can think of some weird workarounds.

One method you could try is checking if the player is in a woodland mansion, and, if they are, having a chance of spawning illusioners with a random offset somewhere nearby. You could check if the player is surrounded by blocks that make up the woodland mansion using the 'check for block in 6*6*6 procedure template, and probably also check that there are evokers and vindicators nearby using the 'does entity exist in radius' function. 

Then, you could use random numbers for an offset, making sure they aren't too small/too close to the player, then increasing the y level until you find a 1*2*1 space of air with a floor below it to spawn an illusioner on.

...Or, if you wanted to make this much, much simpler, whenever you spawn an evoker, or whenever a vindicator spawns and there's an evoker nearby, you could just spawn an illusioner at the same location. This would be as simple as using the 'do with 70% chance' procedure template, and setting the global trigger to 'when entity spawns.' (Though keep in mind this would probably also affect raids.)