To me, the answer is no. They are "employees with equity" but not co-founders. A founder is someone who started the company - period. I have no other definition with which I could be happy with. To call them anything else is misusing the word IMO.
Imagine an investor meeting:
Potential Investor: "So you three are the co-founders. How did you start the company?"
You: "Well, I actually started the company with two other guys. These new guys came along about six months ago."
PI: "So wait - they didn't actually help you found the company?"
It's just "messy" if you try to give them a name that just doesn't apply.