In the “Things” approach, the addition goes into a default bucket, the “Inbox”. To follow this model, you would have to add an “added imports” bucket to Agenda.
In the “Notes” approach, you can choose the “New note” option and are presented with a list of Notes folders. Also, you may append to an existing note. To follow this model, you would need to write more interface code for the Agenda user’s “Add to” step & they be presented with a decision at that point as to where to put it (another step)
For what it’s worth, when I use “Add To” on the iPhone, it is almost always for Things (which I find less unwieldy than emailing stuff to myself). Everything goes into the default bucket and I sort it out later into its project folder when I look at it on the Mac.