There are two ways to get external instruments into Notion.
1) By the built- in presets: These automatically map all articulations, so if you i.e. enter "pizz", the playback will use pizzicato samples. This, as you noted, works for the VSE, EWQLSO and Miroslav. In due course other libraries will be supported like this.
2) By loading a VST (Virtual Instrument) by hand. This works with ALL VST instruments. In this case you have to load the correct VST (say, Kontakt 4) in the Notion VST manager. You then have to load the correct sample set (i.e. French Horn Ens Crossfade) from your library. Then you can record. You can still enter articulation etc , but Notion most of the time will not play it. To make it play back, you have to create a sequencerstaff linked to your instrument staff. There you can enter Keyswitches (which switch articulations), Modwheel data etc....
There is a very nice tutorial about all this by Lubo (Astinov, product manager of Notation) on the Notion youtube page:
http://www.youtube.com/watch?v=ZEdAmh318CoHope this helps, if not do not hesitate to ask.