it can, and has been done. pools are used for ponds both indoor and outdoor. many people, myself included, have used swimming pools for the purpose of fish.
while it is possible to have a tropical pond outside, its extremely expensive for heating, considering most outside pools are tens of thousands of gallons. your only limited by your bank account with an outdoor tropical pond.
however, outdoor pools are easily used for many cold water fish including koi as long as its kept from freezing solid. if it was a swimming pool at one time, some filter conversions, and scrubbing and running with no fish and heavy water-changes and dechlorinators to ensure theres no chlorine residue or leftovers to kill your fish.