I'm looking for an extender/filler for my spice mix
The professional (read: industrial) solution would be maltodextrin powder. It is rather tasteless, has good solubility in liquids, and doesn't change much texturewise. 

In homemade spice mixes, the popular filler solution are dried breadcrumbs of a very small size (like breadcrumbs for schnitzel breading). They are noticeable, because the mix gets gritty, but the consensus among those who do it is that this is a feature, not a bug. They could also slightly thicken a wet dish (soup, stew), but present no problem when strewn over dry food. 

You can also use a nut flour made of nuts without much of an own taste, like apricot kernels or low quality overdried almonds. Again, this will be somewhat gritty, but not as hard as the breadcrumn solution.