Games : How to add blocks into a dispenser without clearing blocks in it

I am making a map and i need to find a way to be able to add blocks into a dispenser without clearing the items that are already in it. I have l looked everywhere/tried everything and i'm afraid that all the time i have used on the map is a waste. Please help me

I have done set blocks, entity data. i don't know what else to do. Any ideas?