Searching iTunes Library for file location?
This won't do precisely what you ask, but it will provide the track reference in the variable "trackRef" from the file location specified by "filepath" if it is in the library.  If the file is not in the library, it will be added.

tell application "iTunes"
    try
        set trackRef to (add POSIX file "filepath" as alias)
        refresh trackRef
    end try
end tell