I heartily second your motion for including accordion registration symbols in Notion. However, PreSonus should be aware that there are many such symbols, both for treble and for bass, and that the reed setup for a particular accordion will determine which symbol set should be used.
I would also ask PreSonus if they could come up with a scheme to enter accordion bass in AAA notation and play the selection back with full chords. I know that they will require an adequate description of AAA notation to do this, and that can be found in a number of places on the Web, or they can ask me or any accordionist.
As for adding accordion sounds to the library, there are many sources of these sounds in many formats. The previous two answers have already given you two sources.