Some wheels have a wide area where there is a 60 degree angle for the acorn lug nuts; with these wheels you can drill the M12 holes to 15mm with enough meat leftover for the acorn lug nuts to seat properly.
For wheels that do not have this advantage, the wheel shop will have (2) choices
1.) Drill 15mm bolt holes while also drilling a 60 degree angles for the acorn lug nuts; they use a special drill bit for this
2.) Drill 15mm bolt holes, then press-in steel inserts that are made with 60 degree angle