Pretty damn near impossible.
A freewheel screws onto a FREEWHEEL HUB
A coaster brake is integrated INTO a coaster hub.
Because of the size and configuration of a coaster hub, there are some coaster to freewheel adapaters that allow you to screw a freewheel onto a coaster hub... But, not the other way around.
If you want a coaster brake, you have to purchase a coaster hub, which is very tough to find in a good 36 or 48 hole variety, then build it into your existing wheel and depending on the size of the flanges on the coaster hub, you may need new spokes as well.