Well, ANYTHING is possible if you have enough money, including modifying your Maverick 88.
I mean, you could have a gunsmith machine your gun (take off the magazine attachment lug on the underside of the barrel), make a new magazine lug if required, attach it towards the end of the barrel, make an extended magazine tube, attach the magazine end to the new and relocated lug...and you'd have a "custom" Maverick 88.
But as you would think, the expense would be more that it'd be worth for a couple of extra rounds on an lower-end (cost-wise) shotgun...so it wouldn't be a good solution. For the amount it'd cost for a GOOD gunsmith to do that job, you probably could buy the other Maverick 8-shot, so there isn't any point of having that kind work done on it. I'm just saying it COULD be done...and by a good professional 'smith, it'd look like it came that way.
Still, I'm not informed about gunsmith labor costs nowadays so maybe it would NOT be that expensive to do -- relocate the mag-lug and extend the mag tube (seems not that big of a deal) -- but you could find out by asking 'smiths in your area.
But as I said, 6 shots is still plenty...if you hit with the 1st or 2nd one you'd still have 4-5 rounds left.
-- John D.
P.S. With an 18.5" barrel, even with an extended mag tube, you might get only ONE extra round capacity...you may need a 20" barrel for two rounds. You'd need to check. If just one round, it'd be even less worth it to do a conversion.
JD