Extrusion can only be applied to vector-drawn objects, eg. Shape Layers. You can't apply extrusion directly to a precomp layer, but you can, of course, extrude a Shape Layer object then precompose it.
Here's what I'd like to do. I want to extrude some text, but I don't want just plain text. I want to spruce it up with some other layers and blend modes (e.g. add a concrete look to the text, etc.). Then I want to extude that whole comp. So I assume I can't do this, even though the text inside the comp is a vector?
Buy ShapeShifter. It operates on Alpha channel contours.