Sorry, if MOSS was installed using the "standalone" option, you
can't extend it or join it to a farm. You will have to create a new
farm, backup and restore any site collections you want to preserve,
then re-install MOSS on that server to join it to the new farm.

A standalone installation  is a good way of quickly getting up a
system for testing, but you are much better off building a single-
server-farm configuration(which can be expanded) if there is any
possibility of it being used for production. The drawbacks are a
more complex install process and need for a SQL Server license.