I have created a custom template that I would like to add to existing projects, but the online guide only seems to indicate that I can use a template to start a new project. Any advice on how to add my custom template to an existing project instead?
Karen
Hi,
Adding a template to a project does not really make sense in terms of words 
do you want to apply a template to an existing project? If yes, this indeed does not exist and there is possibly an existing thread on the topic.
Hope this helps.
Hi @Karen_Peterman, welcome to the Asana Community Forum and thanks for sharing your feedback with us!
This feature isn’t available right now but I’m moving your thread to the #productfeedback category for the time being to give you and other community members an option to vote for this feature! 
There are components that do not alter data such as the names used for custom fields or their values. The same goes for column order and so forth. Other parts can destroy data, e.g. delete a custom field (which is resolved with a CRITICAL warning and perhaps a SETTINGS value of allow/do not allow to apply destructive templates to existing datasets).
Anyhow there is a LOT of good uses as many of these user created UIs (which is what in part a template does) change over time. Furthermore you can LINK templates to auto-update when the master template changes. Perhaps a new template type can be created, UI Templates.
There are many ways to implement this but if you have many users using the same template for a long time and you want to fix an error (e.g. missed a value in a multi-select custom field), now your only option is to go to each one and have it changed manually or start new projects with the fixed template and move all tasks, both terrible solutions if you have 80-100 projects with 8-12 people.
HUGE waste of time.