Do you mean make your regular flex into stiff?

There is nothing to prevent you doing this sucessfully if you take your time with the prep work.

You can either hard step the shafts (I can send you instructions on how to do this) you already have and then you will only need to buy a few new shafts to replace the ones you can't do or you can go with trimming from the tip then adding the required extra length to the butt ends.

Both ways are fine and relatively simple to do.