Quick answer seems to be no.
Create a new mailing, pick one of the Mosaico templates. Add blocks, change text, etc., save it. Now after all that work I thought I'd like to save this as a new Mosaico template.
Clicked on the little save icon next to the Template pull down on the very first screen of new mailing. Entered a name, saved it. It saved it as a regular template, which is nice for the folks wanting a way to save a Mosaico template to use for regular email.
I'll be happy to enter a new issue for this if it's a bug or feature enhancement.
Using civicrm 5.9.1, Mosaico 2.0-Beta4