You CAN make a five-string of sorts simply by drilling into the tenor neck and installing the peg. The string would hang off the fingerboard in mid-air, and the scale length is too short for real tone, but it can be done (I've seen it). The preferred method, of course, is to make the new neck. You can buy neck blanks, from rough-cut to nearly completed, at the Stewart-MacDonald company, for one.

Bob Clayton