I do not know the Pentax system well. You either need a macro lens or a extention tube set or a bellows or a simple close up lens. If you can get a doublet close up lens that fits the front threads on the lens you already have you could get very nice results. I have seen examples made with a doublet close up lens that out performed an extention tube.
You could go to the hpmarketing website and check out Novoflex bellows to see if they offer adapters for your camera. If they do then you could use an enlarging lens on the bellows which is very flexible and the equal of a good quality macro lens.