Gradual ND filter over circular polarizer???

Dear Friends,

I have a Sigma 10-20mm with a circular polarizer on it. I saw a grad ND filter for sale and was wondering how it would work if I bought a adapter. Could I screw on the adapter over the polarizer or would i have to take off the polarizer?

We are traveling to New Hampshire next week and I want to take scenery shots. Would it be a good idea to buy the new filter and use it at times without the polarizer?? What would you do?

Please help as I need to decide in a couple of days.

Thanks,

Zsolt