Import Database

This is a little odd, and I can't quite figure out how to do it.

I have a postgres 7.2.2 database, without the DB_VERSION file, so I'm assuming
this is an "alternate location" - as you can tell, I'm not a DBA.  Anyway,
the machine it was on got hacked, and I backed off the things I thought were
important, reinstalled the OS and put the other things, including this DB,
back into place.  Problem: I don't have the /var/lib/pgsql part of my old
installation, and now I have an apparently-good postgres 7.2.2 installation,
and an apparently-good database, but I cannot get the postgres server to
recognize the old database.

Obviously, without this I cannot dump and re-import, so I'm at a bit of a lss
as to what to do.  Is it possible to tell postgres that a database already
exists, and just to use it?  (I do know the information on the user who
owns the database, and the old passwords and such.)

Thanks

-jeff