Third party wireless remotes work just fine, I once had that one for my K-x, which supports only wireless remotes.
Cost me 6 euros ($7.45) and it could also trigger Canon, Nikon and Konica-Minolta cameras. The downside was, that at one point it began to eat batteries like mad, so I got the original one - Pentax remote control F.
Like stated before, you have to set up your camera to open the shutter on first button press and then close it at the second push, as wireless remotes do not have the button lock that wired remotes tend to have.
K-x also had the IR port only at the front, but I discovered, that if you build a reflector (a piece of tin foil and some cardboard and tape to hold it in place), you can trigger the camera from behind too. Still, I usually triggered it from the front, you just have to be really careful not to step/reach into the view of the lens and not to accidentally touch the tripod (even stepping on the soft ground near the tripod or shaky floor can ruin your shot).