To unlock the musket in edit mode you have to get to expert on archery and complete expert with a rating of PERFECT. Then select Archery again and Magoichi will appear and challenge you. Defeat him and your edit character will have musket. Note: This may only work on the Japanese version of the game.

Magoichi can kill you very easy when he get to low life and he becomes almost invulnerable to all attacks, the best tactic in killing him is to get your self on red life and keep running away from him until your muosu is full, then use your muosu attack on him this should kill him if Magoichi is on low life.