You can do it if you match the steerer diameter and length correctly. The problem arises in that the suspended fork is likely longer than the rigid fork and will raise the front end of the bike. This may adversely affect the fit and handling of the bike. Bikes made for front supension incorporate the longer fork length into their geometry. It may work OK though depending on the geometry of your frame and the travel of the suspension fork.
Would it be worth doing? On a cost basis, probably not.