I've asked about this before and got no replies, but I just found out a way to do it.
-Save the project you want to use as a template as a normal project
-Using iFunBox or any other software that allows you to access your iOS device's files from a computer, go into Cubasis-Documents-Projects
-Rename your custom template's extension from .cbp to .cbt (do it on both the folder and the document file inside)
And that's it. You can also delete demo and template songs while you're at it, but be careful!
I wish Steinberg would give us a way to create (and delete) templates from within Cubasis, it's a useful enough feature. What good is a template for if it's not custom made for your own needs?