[rayjay]

It might be done, but it also may stress the seahorses to the point you could loose them.
Cycling the tank could also take some time depending on your procedure and the rock you select to use.
Main reason though is to learn the specifics of the different needs seahorses have compared to reef tanks.
Just for example, temperature should be 68 to 74°F for bacterial reasons, lower than most reef tanks are kept.
Tank volume MINIMUM would be 30g for first pair, and 15g for each additional pair. Dan Underwood of seahorsesource.com has lately mentioned that by having 30g for EACH pair, even added ones, his customers have had better chances of success.
When you think the tank is properly cycled, challenge it by bringing the ammonia level up to 2-3 and see if it clears overnight. If not, continue cycling.