Yes, it's possible, though I can't recall seeing anything like this. The real
question is why would you want to do that? You can use the security  group
directly without needing to grab the individual users...

It is a request of my security  team: a WSS site  membership must be centrally
maintained by them, using AD security groups. If a member of the site moves to
another team, he is removed from the security group, and should automatically
have his site membership  revoked. But on the other hand, members  of a WSS site
can have different access levels (reader, contributor,...) that will be managed
by a "local" site administrator (a member of the team). The list  of members is
also an important feature: you can use it to know who are the members along with
you, who is online and who is not, directly access members' personal sites or
profiles, ...
When you say "it's possible", what do you mean? How?

I'm still not seing anything that can't be accomplished by simply adding the SG
directly as a member of the site, unless there is somethign I'm missing. Adding
the group  will handle adding and revoking access, and once the user  has actually
visited the site  they will show up correctly in the members  web part.

Sorry to answer so late, but I eventually got a SPS server  installed in a "live"
domain (with AD and all), before that it was a single server with local users.
So I checked what you answered me and it does not work ! I have added a security
group from AD as a contributor to the web site. The group  appears as such in the
members web part. But then I asked a user  that is a member of this group to
connect to the site. After he does that, his name still does not "automatically"
appear in the members  list.
So unless I got it wrong, I still need some tool that allows me to extract
members of a security  group and add each of them to the members list  for my
site.
Any idea (anyone)?