Ive been asked this before. The issue is, Ive never actually seen a set of OEM reflectors. I already had some LED ones on my car when I bought it so im unsure if they're made the same way as the ones im buying.
The ones I have leave a small gap of 2-3mm between the inside of the lens and the back reflective surface, this allows me to insert an LED. If the OEM ones have this gap then I guess its possible, but of course all my references for drilling the LED holes would be different.
As for the wiring, im guessing (I did this, and this is what the fitting instructions show) that most people wire them to their sidelights, but yes, technically you could use a switch to control them manually.