You can use another After Effects layer for a reflection by using the layermapping feature. Essentially, this involves assigning your AE layer or precomp to a layermap number, and enabling that number next to the reflection map thumbnail in the material editor. Then apply that material to one or more face(s) of the object. You'll be able to adjust the amount of reflectivity, just as you can adjust specular highlights, transparency, etc for that material, even after you've aplied it. See the documentation or some of the on-line tutorials for more details about layermapping. Or ask a question here if you get stuck.
Depending on the look you're going after and the 3D shapes you're using, getting convincing results may require some additional manipulation of the reflection layermap.