It is possible. Bethesda even had PC mods running on a 360 so they know it can work.
It will never happen though because of various security concerns that would become a problem if anything were to go wrong during installation. On PC you can modify files yourself to fix any problems, you don't have the same ability on a console.
Until Microsoft and Sony come up with a system that they consider secure and safe it won't happen and I don't think it will happen until at least the next gen consoles
Bethesda did say however that they were interested in expanding the modding community to consoles but that it would take time...... a very long time.