Thanks for the mention, @Bastien_Siebman.
Hi @Amanda-InnKeeper! Asana does not provide this capability. What you’d need is a rule trigger that acts based on adding a tag to a task; and a rule action that adds a set of subtasks based on that trigger. Asana has neither that trigger nor that action.
However, you can do exactly this via an If-Then Rule in my Flowsana integration 
. Flowsana has a rule condition/trigger for “Tag added to task”, and it has a rule action for “Add a set of subtasks copied from a template”. You can see more info about how that action works here.