Welcome to the site !!
OK , the main issue is going to be the 250L has 43mm forks and the KXF has 48mm , so about the only way your going to make anything work is to either "try" to adapt the 450 clamps to fit the steering stem of the L , and hope that the offset will allow full turn radius of the L when the KXF clamps touch the frame stops on the L and clear the radiators , etc .. , there are a lot of variables that need to be accounted for , before busting into this venture , "if" its possible , its most likely going to require some sort of machine work and a lot of trial and error , heavy on the error part
you would also have to consider the overall length of the forks and deal with the axle offset , because "if" the KXF fork is longer , then you will have to limit internally the KXF forks , to get the balance front to rear correct , otherwise you will inherit all sorts of handling sorrows that could prove dangerous if not addressed properly , then you will need to make sure the offset of the lugs does not cause other handling issues , by either too much rake , or not enough , since most likely the frame where the head stay is , would most likely be a different angle than the KXF
So to somewhat answer your question , it "may" be able to be done , but i would figure that without major modifications , its not going to work real well and might be a lot of wasted effort , if handling or comfort is your goal , i would try re-valving the L forks , if strength and jumping ability is your goal , you would be better off getting a bike more suited to that type of riding that is already set up to do those things from the factory , as the L has its purpose and that is primarily street and light duty use , and the KXF is definitely not valved for street use , and it would most likely beat you to death "if" you could find a way to make them fit properly if at all...............hope that gave you some answers you were looking for ....