Many aftermarket rims are manufactured as "blanks" which get drilled to order or to demand. So yeh its possible you can get a 5 studded rim of the same design. But to modify a 4 stud to a 5 stud, that would compromise the strength of the rim.

I wouldnt trust the strength of those one-fits-all multistudded ones, they seem a bit dodgy.

Your best bet is to sell the 4 stud rims and buy the equivelent or similar 5 stud rims.

What brand and model are the rims?