The answer is yes. One could, but it is not recommended. Stainless requires relatively high forces and pure copper without alloy strengthening additives does not have the strength at the temperature generated to hold up. Mushrooming will occur quickly. Weld nugget failure will be frequent and require constant electrode dressing and attention.
My answer is no don’t use pure copper. In another blog article electrode selection for stainless steel was addressed:
WHAT IS THE PROPER ELECTRODE MATERIAL FOR SPOT WELDING STAINLESS STEEL?
Stainless Steel weld schedules are available in:
AWS Specification C1.1 Recommended Practices for Resistance Welding
In this specification the data necessary to dial in a good starting weld schedule is available
Reference: AWS Specification C1.1 Recommended Practices for Resistance Welding
RWMA Resistance Welding Manual 4th Edition