hope this question makes sense

Imagine you extrude a cylinder.  Then you create a plane that intersects the cylinder at a 45 degree angle to the axis running down the center of the cylinder, then draw a circle (larger than the cylinder) on that plane.  Now, you want to extrude that circle, but not in a direction normal to the plane the circle was created on, but rather you want to extrude it the direction of the axis of the cylinder.

How would you do that?

I realize I could draw another circle in the same position below the first and do some kind of loft between them - but let's say it's not an easy-to-draw thing like a circle, and maybe is some arbitrary geometry that is not easy to redraw... and you just want to somehow extrude the original sketch?