I think the problem is that there isn't that much latex in the paint, it is
mostly water.
If the paint is free it might be worth a shot but I wouldn't count on it
working. You might find out there is more than one kind of "latex" or there is
something else in paint that compromises concrete.
A lot also depends on the property you desire from your admix.
ASTM spends a lot of money assuring the result you get with specified products.
Anything else is a crap shoot.
It might be noted crap shooters win about half the time tho ;-)