drilling SMT footprints for fitting THT parts
I totally agree with others that drilling into the pads is a very bad idea; even if there are no conductors beneath, it will most probably lift the remaining parts of the pad. However, I'm quite surprised no one mentioned that THT component leads can simply be bent to turn the component into makeshift SMT component. If the board is part of a small quantity batch or a prototype, I would just simply do this:

 

More info on this here.

Of course, if it's a large product batch, I agree that cutting losses and doing it properly with SMT components will prove better in the long run.