scarborough new york? i stop at that station everyday going to my internship in Dobbs Ferry. i have a 2x pentax converter but i use it on my k1000 so i don't know if it'd be the same for digital.. i would imagine maybe. i'm sure they're not expensive. try bhphotovideo.com
I can't speak for Pentax glass as I shoot Canon, but I own the Kenko PRO 300 AF 1.4x Teleconverter and love it. I have shot both Canon's and Kenko's 1.4's and can not tell a bit of difference in IQ. The only difference seems to be the color of the paint on the outside.