You should be able to do so because most domestic refundable fares are one way (or point to point), not RTs with requirements. To be sure, since there shouldn't be any change fees if you're buying FULL Y or FULL F, you could ticket it as two one way tickets on separate PNRs.

However, be careful to check fare rules very carefully. Some refundable fares now are refundable only with a fee and may also have change fees.

For international, the rules seem to depend greatly on your departure country or point of sale.