Hey & welcome to UCS in general and the forum in particular.
Is it possible to add a domain share on my UCS server that is on another server/s (Ubuntu and Windows)?
It is possible, though not centrally managed. In Univention's Management Console (the web-based administrative frontend) you only create shares for Univention servers, not for other servers. Instead you create the shares on the respective non-UCS servers themselves.
The easiest is creating shares on a Windows machine. You only have to join the Windows machine into the Active Directory domain provided by your Univention DC Master server. Then you can use the usual Windows-based method of adding a share: right-click on a folder in Windows Explorer, select "Sharing", make it a share and optionally restrict the users & groups that have access to it. As the server is joined to the UCS AD domain, you can select AD users & groups there.
It's more complicated for a Ubuntu machine as joining such a machine's Samba service to the UCS domain is possible but involves several manual steps. There's documentation avaialble for it. After a successful join you can configure Samba on your Ubuntu machine to provide a share.
There's an alternative to joining that Ubuntu machine to the domain: let the Ubuntu machine provide an NFS export for a certain directory (e.g.
/data/bigdrive), mount that NFS share on one of your Univention servers (e.g. in
/net/ubuntu-server/bigdrive), and then use the Univention Management Console and configure a regular share that uses the mounted NFS share
/net/ubuntu-server/bigdrive (make sure to make such a mount permanent, e.g. via
/etc/fstab). The drawback of that method is that your Ubuntu server won't be able to resolve user & group names to IDs and vice versa if it's not joined into the domain. This means that accessing files & directories in
/data/bigdrive directly on your Ubuntu server will lead to problems with access permissions.