Customize my PHP based POS with an extra feature

I am currently using a POS called "PHP POS". I would like to add an extra feature or two to it, as at the current moment it does not offer everything we require to run our business smoothly. I would like 2 things added: 1) The main thing we need right now is a way to track "Buying" pre-played games from our customers. The way I Propose this is done is by adding an extra tab to the "Receiving" section of the POS and call it 'clients'. When under this tab we can select a customer or create a new one to receive from. We then add the game and the amount paid as per usual. Instead of paying cash, the total will get added to the customers account as credit. The Receivings, Credit and Customer sections are already present in the POS - we just currently can only select suppliers to receive from and need a way to receive from our customers as well. 2) This is a VERY small tweak we would like done. There are multiple stores in the PHP Pos and it currently only shows the qty for the store you have selected in the item list. I would like a single column added that simply shows the Total qty of that item, to make it easier to see at a glance what is sold out. I can provide more info on request.