The verb 'to do' can be used to emphasize things, e.g.: I do understand it.
Can it be used to emphasize a sentence that uses the verb 'can'? E.g.: I do can play football.
If not, how could I achieve the above?
The matter is one of grammar. In the context you are talking about, do is acting as an auxiliary in the have does. The problem is the use of a double auxiliary. This can happen with the tense and voice auxiliaries, have and be, but not with ‘do’.
Even here there is an exception. Somerset dialect, now sadly moribund, permits
“I do go to the gym every Tuesday” or even
He do be looking for the way to the chemist.
But this is seldom heard today. It was educated out of children by the school system years ago.
So instead of I do can, you can can say I really can
No, it depends on your intent. You could say “I can do football”, with a literal meaning being that you are capable of football and an implied, more colloquial meaning that you are capable of (joining) football. I don’t know if there is a deeper, more exact rule, but as a native English speaker that’s how you’d sentences akin to this one.
