This isn't something new, there are a bunch of threads in the forum discussing how to implement multisessions on cd/dvd through NeroCom but none of them are very readable, and there are no .net examples.
How do you import existing tracks properly into the new track you are trying to burn? What I'm doing right now is taking the NeroFolder parameter from OnDoneImport2 event and setting it as the RootFolder of the NeroIsoTrack object I intend to burn. If there are no existing tracks on the media then I do not bother importing.
The result I'm getting is that I'm overwriting previous tracks and so only the last burned track appears on the CD/DVD in windows explorer.