Yes.
K-7/K20D/K10D:
1) Custom function "Using aperture ring" must be set to "2 Permitted".
2) In manual exposure mode "M": Set the lens to the desired aperture, compose the shot, press the green button (this will set the correct shutter speed), shoot.
K-x/K-m/K2000:
1) Custom function "Using aperture ring" must be set to "2 Permitted".
2) In manual exposure mode "M": Set the lens to the desired aperture, compose the shot, press the green button (this will set the correct shutter speed), shoot.
K200D, K100D, K110D:
1) Custom function "Using aperture ring" must be set to "2 Permitted".
2) In manual exposure mode "M": Set the lens to the desired aperture, compose the shot, press the "AE-L" button (this will set the correct shutter speed), shoot.
*ist DS, *ist DS2, *ist DL, *ist DL2:
1) Custom function "Using aperture ring" must be set to "2 Permitted".
2) In manual exposure mode "M": Set the lens to the desired aperture, compose the shot, press the "AE-L" button (this will set the correct shutter speed), shoot.
*ist D:
1) The firmware must be 1.10 or higher.
2) Custom function "F stop other than A" must be set to "on".
3) In manual exposure mode "M": Set the lens to the desired aperture, compose the shot, press the green button (this will set the correct shutter speed), shoot.
Note that K and M lenses are manual focus lenses.
Another method is to engage the optical preview (stop down) button in manual exposure mode. This turns on the lightmeter and the exposure bar. While keeping the optical preview engaged, adjust aperture or shutter speed until the exposure bar shows the correct exposure or shows over/under exposure to you liking. Also when using this method the "Using aperture ring" custom function must be set as described above.
From 'Monopod': It is also useful to set the body in MF (manual focus) mode
otherwise the shutter will not trigger unless the target is in focus.
See also:
https://www.pentaxforums.com/forums/pentax-lens-articles/110657-how-use-manua...7-k-x-etc.html