Can fill in a doorway opening in a foundation wall over the top of the slab?
I understand the question to be about filling in the foundation wall through the former doorway opening over the top of the floor slab. I would not do so because the slab will have a tendency to move with seasonal soil movement, etc. 

Presuming that the slab is poured over a lowered area in the foundation wall, I'd cut back the slab and pour your foundation wall repair directly on the original wall. This will provide the most stability. 

If you'd rather not do that, consider simply framing in the opening with wood, using pressure-treated lumber where there's concrete contact. Construction adhesive would secure everything just fine. If it's exposed below the siding on the outside, paint it with an appropriate gray paint to match the foundation. Caulk it in and it'll become nearly invisible.