Plucker support in Calibre has been on my wish list for a long time but it will probably never happen. The only alternative that I have ever come up with is to use FBReader. You can open plucker books with it then copy and past the text to a file the use Calibre on that text file. It's ugly, painful and cumbersome but it can work. It's enough of a pain that I just use
FBKindle now when I want to read some of my old book.