Most inserts have a shoulder on them. If you put a washer over the nose of the insert, that fits down onto that shoulder, that shortens the amount that the insert can enter the cavity of the mold, giving you more lead in the bullet, and a more shallow cavity. The thickness of the washer will be the equivalent to grinding, or cutting the nose down on the insert that same amount. Adjust the thickness of the washer(s), and you get more/less lead in the cast bullet.
If the insert doesn't have a shoulder-- hmmm-- you could make a thicker washer that has a hole drilled and tapped from the side of it so an allen- type screw can be used to lock it to the shank of the insert.