Yes extension tubes are legal on pump shotguns. However, one is
limited to 10 rounds max in that pump action shotgun tube magazine.
Sec. 53-202w. Large capacity magazines. Definitions. Sale, transfer or possession prohibited. Exceptions.(1) “Large capacity magazine” means any firearm magazine, belt, drum, feed strip or similar device that has the capacity of, or can be readily restored or converted to accept, more than ten rounds of ammunition, but does not include: (A) A feeding device that has been permanently altered so that it cannot accommodate more than ten rounds of ammunition, (B) a .22 caliber tube ammunition feeding device, (C) a tubular magazine that is contained in a lever-action firearm, or (D) a magazine that is permanently inoperable;
On a side note, there is further limitation on tube magazine capacity for post 9/13/94 manufactured semiautomatic shotguns in the
old AWB, which still applies. In that specific case, if the post ban semi auto shotgun has a tube magazine in excess of 5 rounds, having any additional evil features (a folding or telescoping stock or a pistol grip that protrudes conspicuously beneath the action of the weapon) would make it an assault weapon.