There is a tab on the template-editing screen that is labeled as Other. This allows you to add raw text for the hypervisor to interpret.
For example, for running a VM with Windows that will have Windows recognise and use 8 cores, you can add the following:
<cpu><topology sockets='1' cores='8' threads='1' /></cpu>Further, you can add Custom Tags, which are no more than ⟨key, value⟩ pairs that will be included in your VM’s metadata. You can use them as you wish.