How can I accomplish different Quantity Discounts for Group Pricing customers?

I have Zen Cart 1.3.7 and for some products I use Quantity Discount (Admin > Catalog > Products Price Manager)
and I assigned some customers to Group Pricing (Admin > Customers > Group Pricing).

A product is listed for all customers as:
Qty Discounts Off Price
1-5: €20.00 (normal price)
6-10: €15.00
11-20: €10.00

Customers who are assigned to Group Pricing receive 10% discount.
I am looking for a way that customers who are logged in and assigned to the 10% discount Group Pricing will get a different Quantity Discount, e.g.:
1-5: €20.00 (normal price)
6-10: €17.50
11-20: €15.00
(and 10% off the total price, which is accomplished by using Group Pricing)

The Quantity discounts above are just an example.
In the shop they will differ for every product.

Is it possible or do I need a modification?