I have a MOSS 2010 server nicely configured but its a workgroup / stand alone setup.
I want to add the computer to the domain. Will this break the sharepoint installation?
It will not break the installation, but you may want to consider creating a new farm. It is possible to do a migration, but given the number of service accounts that have to be reconfigured it would probably be easy to configure a new farm and re-attach the databases to bring in the content.