Can you create a new Journey from existing Templates using the Journey Builder API?
As far as I know not directly.
You can, however, simply
create a 'real' journey from each of the templates and save them without configuration or only those you always need the same way.
Name those with a syntax that indicates those as templates
move them to a folder for your journey templates
retrieve those journeys via API
Use the retrieved journey configuration for your API based journey creation