shopper group & other discounts

[Piszi]

Hello,

I want to know that is it possible with prestashop to seperate shoppergroup and other (for ex. monthly) discounts?
What I'm trying to say is: if I have a group with 10% off and than I want to discount a product with a value for ex.: 15$ in this case I will not have the 10% group discount.

For example:

I have a product which is 100$ without any discounts
than with 10% sg discount the price will be 90$
but let's say it is christmas and I want give 15$ off from the product than the price I want to see is 85$ and not 75$ (90$-15$)

So is this possible in prestashop? Or is there any modifications which can do it?