psyprofessor,
Fishermen eventually learn that more fishing lures are intended to catch fishermen's dollars, than to catch fish.
Firearms often enough work the same way.
Now that I've politely told you there's a hook in your jaw...
YES, you can take off the pistol grip and replace it with a conventional stock. And, if you want to consistently be able to hit anything with that shotgun without a disproportionately large amount of training and practice, you will do precisely that. Mossberg will be happy to sell you a conventional stock for it, and you can change it out with ordinary household tools.
On some rare occasions pistol grip only (PGO) shotguns are useful. Breachers who carry them as 'master keys' have employed cut-down PGO shotguns for years to blow open doors. They are employed by at least one armored car company I have heard of for use from inside the vehicle where space is limited. Outfitted with a laser and given lots of frequent practice on the part of the user, they can be practical for defensive purposes in the home.
But there is insufficient disadvantage to a full conventional stock on a shotgun in 99% of situations I am familiar with to give a PGO shotgun enough of an advantage to make it a worthwhile tradeoff. Proper training is a must with any defensive firearm, no less so a shotgun; proper home defense training is equally necessary to avoid potentially fatal mistakes.
The idea that creeeeeping ninja-like through your house in search of that elusive bump in the night is a proper course of action is as popular as it is misguided. The idea that such things cannot be done with a full stocked shotgun if absolutely necessary is equally misguided, though less popular.
Please get yourself a copy of Louis Awerbuck's
The Combat Shotgun (
http://www.paladinpress.com/product/40/25 ) and consider taking the NRA's Personal Protection In The Home class (instructor locator at
http://www.nrahq.org/education/training/basictraining.asp ). If there isn't a class near you, the video is available at
http://materials.nrahq.org/go/product.aspx?productid=ES 26840 .
This stuff is NOT instinctive for most humans- it requires education, training, practice. Please get some before you need it...
lpl