Possibly.
If will depend on the style.
Can unworn swimsuits be returned to the store? If so, I would purchase it and take it to someone who specializes in ladies alterations--it's a bit different than mens alterations, particularly in this case. Ask if they think it is possible.
Biggest issue is ladies RTW isn't designed to be altered. Sometimes it's possible, sometimes not.
If fabric can be added, take the swimsuit to a fabric store and look for swimsuit fabric. Don't even think about any other kind! It won't hold up to the punishment. Coordinate a solid or print as much as possible.
If it can't be returned, I wouldn't waste the money on it--just look for something else.
