No reason why not.  But they will have to be sealed with a gloss coat as if you putting the decal and nay flat paint surface.  The issue would be that when the pastels are sealed, it can sometime drastically change their appearance.

Decals first. White wash was applied over a vehicle as a temporary measure to help hide a vehicle during winter. The crews would use care to leave those markings which would help identify them not so much that the enemy could identify them, but so their own wouldn't shoot them by mistake. If some of the white wash happened to get slopped on or oversprayed  on those marking they would clean it off.

When you apply decals over pastels, you'll find that you'll have the fine grains of the chalk under and preventing the decals from laying down. Thus silvering. You'll also wash much of your weathering away. You want to avoid handling the model after the pastels are applied. You also want those marking weathered as well.