Yes, you can definitely use an adjustable ring with a penis pump. The best way to use them together would be to place the ring around the base of your penis prior to placing the pump on. Keep the ring very loose so you have room to grow.
Once the ring is on, place the pump over your penis and pump until you have achieved the results desired. Once you are fully erect, tighten the cock ring down with the adjustment feature and remove the pump. You should now be ready to enjoy sexual pleasure!