I'm looking for a way, while on a node page, add that node to an existing entity reference on another node.

Essentially I have two content types, Task and Task List. Task List uses entity reference to let users create lists of other user's/their own Tasks (There is additional user generated content on the Task List page which is why I'm not using flag lists). I need a way when viewing a Task, to let users add that Task to one of their existing Task Lists.

I think this would be a great feature for users who want to easily create groups of existing content such as top 10 lists, creating collections, etc.