As far as I know there is no way to add drainage holes to an object with infill. It would be nice to have the infill function directly added to the hollow function, in order to add holes afterwards. This would also allow to print several objects at once, some with infill some without or with different infill settings.
There is a trick/bug that can get you infill with holes (most) of the time.
Import your model fresh (Chitubox makes permanent changes to the model state and those can impact this)
Select Hollowing and hollow the model out (I did 2mm). Then poke your holes.
Make sure you have infill turned on (I did 2mm 20%). Slice your model.
Thats it. Holes, and infill.
Many things can prevent this from "working". If you duplicate the hollowed model or you save/load it, then the "hollowed" state will become permanent and it will assume those hollowed sections are by design and you don't want to fill them in.