There are a few ways to do this:
Since you mention mounting a tool in your tool post you can:
1. Set your lathe up to follow a pattern. Member More4Dan has done this and posted some pictures of his setup.
2. Make a form tool, i.e. a tool that has the negative profile of the shape you want and just plunge it into the work. It might work OK for something as small as a section, but nothing larger.
3. If you are really coordinated you can turn both the carriage and cross-slide cranks in unison to get the shape that you want. Sort of like patting your head and rubbing your belly. Brian Gray of Edison Pens told me that he actually did that on prototypes to shape barrels and caps with the aid of a metronome.
4. Convert to CNC.
5. If you keep your shape really simple (a straight taper) you can do most of it with the compound adjusted to whatever taper you want. Of course you'll still have some had work of some sort to do.
Two other alternatives:
1. Buy or fabricate a tool rest and turn with gouges and skews just as you would on a wood lathe.
2. My favorite option is files. you can use them to do the whole job or just for finishing, and they won't get away from you. They are really great for refining shapes.
Hope this helps,
Bill