Can I take a design that is a fill stitch and turn it into a centerline? I am not creating it from an image so click-to-center isn't an option. I have the same question for a ttf letter. If I insert a ttf letter, is there a way to force it to a centerline stitch instead of the standard fill stitch?
Fills are closed shapes. By "centerline" I'm guessing you mean something like a satin line. If so, no. To understand why, you need to understand the different between closed shapes and open shapes ("lines"). The type of shape/object you have determines what types of stitches can be applied. Closed shapes get fills (or outines around the perimeter); open shapes only get outlines.