Observations can’t be flagged as introduced, rather the taxon can be set to introduced in a particular area. Not everything that is introduced is currently marked that way, so if you notice something that should be marked, you can change it. Likewise, occasionally something gets marked introduced when it shouldn’t be.
Observations of introduced taxa can get to Research Grade just like any other observations – they need community agreement on the ID, a date, a location, etc. Here’s one of yours at research grade:
See more discussion about the timing of introductions here.