In answer to the original posters question you can do what you are suggesting but you will need some different magnets.
Cheaper single coils have a ceramic bar magnet attached to the six steel poles. You could replace this with an alnico magnet but not a standard humbucker type. The magnet needs to have it's poles on the 7mm x 63mm face, a standard humbucker magnet is magnetised so that the 2mm x 63mm face on the side is the pole. It would work, just not very well.
Jon