Since all a Staff of Striking did was increase melee damage, why not classify it as a magic weapon - a Quarterstaff of Striking? Then you just need to add a mechanic where it gets bonus damage from charges.
Quarterstaff of Striking: This +3 quarterstaff has the ability to augment the power of its strikes. Along the length of the staff are 25 iron bands, each representing a charge. For each charge expended after hitting a target, the staff inflicts 1d6 of force damage (similar to a magic missile spell) in addition to its normal damage. A maximum of three charges may be expended per strike, and a band shatters and falls off the staff for each charge used.
Caster Level: 3rd; Prerequisites: Craft Magic Arms and Armor, magic missile; Market Price: 37,350 gold.
Design Notes: This is basically a +3 quarterstaff, with the cost of a magic missile wand added in. I used 1d6 instead of 1d4+1 because its the same average damage, but easier to keep track of.