First, welcome to the QB.
To answer your question; Maybe yes and maybe no.
IF your 15-91 has four threaded holes in the right end of the housing, under the potted motor housing, it can be converted. If not, it can't be converted.
My 15-91 and some others I've seen have two mounting holes for the potted motor, and two slightly smaller holes for the belt guard / bobbin winder of the external motor version.
If it has the 4 holes you'll need a spoked hand wheel with stop motion knob and clutch washer, a BG / BW assembly and the two screws to attach them to the head. I've been told those two screws can't be found for sale, but when I did my experiment on my 15-91 I just used parts from a 15 Clone and they worked perfectly.
If you can find the two screws, Sew-Classic has the hand wheel and belt guard.
Joe