In short, no. You cannot port a sealed box.
Sealed enclosures require a specific range of internal volume, as determined by the T/S parameters of the driver(s) for which the box was built. A ported enclosure also requires a specific range of internal volume for the driver, but the required air space in the box for a ported enclosure is substantially larger than that of a sealed enclosure.
First, to port an enclosure, side from the required internal volume of air, the port has to be unobstructed at both ends for a distance of at least the port's inner diameter (if the port is round) or the port's height, if using a slotted port. A sealed box is small enough that, with the driver's own displacement considered, there would not be enough room for an unobstructed port in most cases.
Next, if the driver doesn't have enough volume in the box (after the driver's displacement is subtracted from the internal box volume) the result will be an overly-damped driver, which will yield a peaky response that lacks deep bass and high output, because the driver's movement will be restricted by the lack of air volume for the sealed chamber.
For more on sealed enclosure size calculations, please refer to The SubWoofer DIY Page on sealed box calculations.
For more on ported enclosure size calculations, please refer to The Subwoofer DIY Page on ported box calculations.
