No, they can't. they are microlight hours, not light aircraft ones.
However:
If you fly loads of hours in a C42 Ikarus, a Eurostar or the like - the microlight hotships that also come as light aircraft depending on their paperwork - then you should have no problem doing a Certificate of Test in a light aircraft to keep your light aircraft licence valid.
This then avoids the 12 hours etc, and keeps the light aircraft licence valid - which allows you to fly the microlight!
However, you will still need the full medical if you have a JAR licence,since the NPPL medical is only good for an NPPL. In other words, the medical goes with the licence you hold, rather than the aircraft you choose to fly under that licence.
Hope that helps, it is a common question!