I'm considering buying a steel cross frame, but hesitant because the rear will only accept a true 30mm wide tire. (I won't be racing, not concerned with UCI rules)

The seller tells me that there is ample clearance at the rear of the seat tube and between the seatstays to allow for much wider tires.

Would it be possible (without framebuilding/specialty tools---I just have basic household stuff) to dimple each chainstay about 2-3mm? I'm not too concerned with damaging the paint on the inside of the stays but don't want to do any structural damage or screw up the alignment.

If possible, any suggestions for how I should go about it?