Do not "extend" the port. That'll change the tuning frequency of the port in your sub. Also, by placing it in the cabinet, you'll create an additional "port" length, by effectively creating an additional path for the port (or driver) to pass through, like a folded horn arrangement. You're almost guaranteed bad results with a ported sub in a cabinet. And as others said, vibrations will be a buzzkill too.
For an in-cabinet sub, look to a sealed arrangement, so only the driver is facing through the cabinet grill.
Why do all of my threads suck? Is there an internet posting school somewhere? -wes k