Magento

So my company sells T-shirts, and the way our system works is that each shirt design has an Item-Number, a master number, and each size of that design has a sku. I have been playing with magento for a little while and I can’t figure out how to change this so my configurable product has an Item Number instead of a sku. SKU is a required field and I can’t get rid of it. I know I could just put the Item number in the SKU field when I create the configurable product, but I am afraid that if someone else in my company decides to enter products that they may get confused.

My other problem is that I need this Item Number to also show up on invoices along with the SKU of that size t-shirt. I have no idea what or where I would have to modify to get this to work.

If anyone has any suggestions I would appreciate it much.

Thank You