I haven’t copied GPOs like that myself yet, but there are a couple of posts about copying GPOs between Windows-based Active Directories. As you use the standard Microsoft tools for creating GPOs with UCS, too, it may be worth your while to give those methods a try:
About the users: I don’t know an easy way to copy only certain users. If it’s just a handful then I suggest you create them (and the groups) manually. If there are a lot of users and groups you can also script this with the help of the “udm” command-line tool (this is the “Univention Directory Manager” tool which exposes the same functionality to the command-line that the web-based Univention Management Console does). See the admin docs for more information.