Open Premiere sequence in existing Audition session

I don't think you can do it quite like that; the only way to link a Premiere sequence to Audition is to use the 'round trip' method. The other problem with it is that even if you open the video in Audition in a session, you won't be able to save the result, as Audition will not do a video save any more. But if you start from Premiere, then use the 'edit in Audition' option, then your audio and video will show up in the round trip session, and you can add whatever you like to that, and then return it to Premiere so that the result can be stored with the video. If you want material from another session to add, then you have to open the other session too, and copy and paste what you want. It should work fine, though.