Yes, possible but remember the OEM amplifier in the '97 was only a 4ch x 40W per channel amp. So needless to say, it's not gonna have a whole lot of extra power available for an 8 or 10" sub. So, it will probably be much easier instead to get a self powered sub and use the unused 18W amplified outputs from the headunit for the sub's signal. You may have to add a line-level converter if the self powered sub doesn't have one built in though.
Or you could look for a 6x40 amp (from a later Boxster or 996) and then use the empty channels to drive the sub but most people who have one of those use it to drive rear shelf speakers.
Another thing to consider if you haven't done so already is to upgrade the door speakers which are (using the term loosely) the woofers in the Boxster's sound stage. Very loosely as they are just 5.25" drivers (which w/o a ton of power can't produce squat) in tuned ports. You can fit 6.75" drivers or 6x9 drivers in the space provided by making custom mounts and losing the tuned ports.
Just some things to consider and do a search as there's lots of great info regarding this topic here as it's been discussed (and overhauled) many times.
Good luck
