See my Pentax K1 wish list at B&H Photo.
See June 6 2016 update.
See also Pentax K1 Full Frame Lens Selection.
Lou F writes:
Do you have any idea yet if the K1 will accept my 645Z Pentax lenses via an adapter? If yes, which adapter?
My main concern with medium format adapters is the stress of putting a large and heavy lens on a fairly long lever arm (the adapter), resulting in excessive torque on the lens mount, including possibility of sag (tilt). For that reason, an adapter without a tripod foot isn’t a good solution in general, but there are exceptions for relatively smaller and lighter lenses, such as the Pentax 35mm f/3.5 (about 650g).
The flange focal distance of 45.46mm for Pentax K is ample given the 70.87mm flange distance for a Pentax 645 lens. In other words, there is a 25.41 inch delta—plenty of space to insert an adapter, and one with a robust tripod foot too.
At B&H Photo, the Pentax 645 Lens to Pentax 35mm K-Mount Body Adapter "K" is listed as “No longer available”, but it is listed as in stock at Amazon: 645 Lens to Pentax 35mm K-Mount Body Adapter "K" @AMAZON. A google search turns up various other options.
The Ricoh Imaging site has an Adapter K for 645 Lenses page and various other Pentax K adapter question and answers. But it’s not clear whether the item is still available for purchase. Perhaps that will change given the release of the K1.
There are many adapters for lenses to Pentax K mount. B&H Photo lists a Pentax 67 Lens to Pentax 35mm K-Mount Body Adapter and Pentax Mount Adapter K for screw mount lenses. There are also lens adapters for Contax 645, Bronica PG, Bronica SQ and Rolleiflex 6000 to Pentax K.
Mount conversion is another option: Contax SLR and Leica R and Zeiss ZF.2 lenses can be converted to Pentax K mount via lens mount conversion at Leitax.com.