You do already have extra hands... But they're hard to put socks on. [If Sans can figure out how to make socks stay on the ghost hands, they could finally be visible. As is, not much good for socks. Seems likely any future legs Sans might sprout would also be ghost limbs. And as for the idea of growing more tangible limbs...]

...I could have extra feet any time I wanted. [He says this with a bit of wonder. As much as he complains about being a robot sometimes, the sudden sense of power this brings is considerable, and obvious in his voice.]