I know you can turn a coaster version into freewheel using a "free coaster" but I doubt you can do it the other way round like you're after
...... well not without ripping the old hub out and bolting a new one in !
I reckon best bet would be to buy a coaster version and sell your others if a coaster is what you really want
What's up with good old fashioned calipers with a good hearty squeeze of the brake lever
I've got black tuffs with black 901's and black copy tuff pads on my cali - it's not exactly like droping anchor but they certainly stop me quick enough without me crapping myself
Are your brakes set up properly ? You or a previous owner haven't cleaned / shined the tuffs with any products have you
Could be a slippy braking surface