why not just start the player with 200 wp in throwing?
The hard way would be to check everytime the player uses the attack key, or when an enemy is killed if it's the player. In either case, you would have to check if the player has knives in his hand, then give them bonus thrown WP. This will not work for the last thrown weapon you throw, unless you set up a global var to track when you last used a throwing weapon.