Q. I just want to know if it is legal to continue a series of books that the author has pretty much said the he/she is planning on writing anymore books to the series. What I want to know is: Is it possible for me to start my own series off of a series that has already been published?
A. Basically, no.
You don't own the characters so you can't do anything with them. What you're talking about may be "fan fiction," but this is unclear. Unless you're trying to sell a Star Trek or Star Wars novel where various people write books in that universe, then this is not a route to go. You can't sell such work, and no one will look at it. The only place this is appropriate is writing a sample episode for a TV show in hopes of being a staff writer on that show or a similar program; but in those instances, the show is alive and well, not dying or dead as you described.
Your best hope is to sell an original book that is successful, and then, when you have the ears of important publishers, you can bring up possibly continuing a dead series. At that point, it's a super small chance, but at least that's better than no chance.
Who HASN'T wanted to sit
down and starting writing the
next Tron movie? Alas, if
only it were that easy...