there are no UCR variables for the things you want to include in the interface configuration. However, you can create your own snippet of interface configuration and register it with the Univention template mechanism. That way the snippet will be copied verbatim into /etc/network/interfaces.
Here's a terse guide how to do that:
Create a file in /etc/univention/templates/files/etc/network/interfaces.d, called e.g. 99-mybridge. Its content is the interface code you want copied into the resulting interfaces file.
Next create a file in /etc/univention/templates/info called something like my-custom-files. Its content should look like this:
Last execute "ucr commit /etc/network/interfaces".
The first step creates a template snippet for Univention's templating system. The second step registers that snippet so that it'll get copied into the output file. The last step re-creates the interfaces file from the templates.
Now you can use that interface (well, you may need "ifup br64", too).