PrestaShop 1.5 special rules

Hello , we need to modify our prestashop 1.5 with the modification "Special customer groups with negative reduction (Charge)" Details: We need to have special customer groups that will have negative reduction (charge/fee) and will take prices before reduction from the price rules of another group. Example: We have wholesale group (WG) with tax excempt modification (override). For every product , we set up before the special prices (special price tab in product view) regarding to the WG. We need negative reduction customer group (NRG) that will take prices from special prices of WG + reduction. Simply lets say that we have product with special price for wholesale group (setted up before on special price tab in product screen in back office). That price for WG is 5$ (Tax 0%). Lets say that we need special negative reduction group (NRG) with 20% negative reduction of WG price. Formula is: NRG price = WG price + neg. reduction 20% or (exact values) 6$ = 5$ + 1$ Reason We need this modification because we have set up the WG prices and now we need the NRG groups because its difficult for us to set up special prices for the NRG groups in the product screen of backoffice. IMPORTANT: We are looking for experienced person with profile background/history. We meant experienced person who was making exact or similar modification before (or SKILLFUL PrestaShop programmer who is making modules etc). We dont look for a experimentators, scientists or "Try or failure" people or wannabe prestashop programmers. Thanks for offers MagiCam