If you have a double sink you can often gain a few inches by changing the center waste kit for an end waste kit, or by reversing an end waste kit to put the discharge on the opposite side.

Code expressly prohibits installing an extension on the P-trap.
If you cannot achieve the desired length in the manner mentioned above you will need to extend the waste arm coming out of the wall however, be very careful here. There are very strict limitations on the overall length of the waste arm. Before you extend it you would need to know the current length of the pipe from the trap weir to the point were the waste arm connects to a vent in the wall.