I'm not sure what your question means, but it could mean one of two things. Here are the answers:
1. Yes you can put a loopback interface into area 0, no problem. That way non-zero areas can talk to each other.
But if that is your only interface in area 0, and you are doing it on more than one router, then do not expect the area 0's in the different routers to talk to each other. You will have what is called a "discontiguous backbone", and the way you get round it is using a virtual link. So that brings me to my second alternative answer:
2. Yes you can extend area 0 into a router by creating a virtual link through a non-zero area (but not a stub or an NSSA). To do this, look at the area that you want the virtual link to cross, say area 51, and do area 51 virtual-link on the routers on each end of the virtual link, where is the RID of the router at the other end of the link.
Interestingly, if you are extending area 0 into an outlying router using this technique, it does not need to have any other interfaces in area 0. The virtual link is sufficiently "area 0" to get the other areas talking to each other.
Kevin Dorrell
Luxembourg
