Not advisable unless you're goin for experimental results. In that case, just pull it up in ACR and use it.
You cannot select a 1Dx profile from the ACR profile dropdown for the Pentax, as it won't appear there - as deeejjjaaa noted, you need to edit a donor 1Dx dcp file and replace the Canon CameraIdentifier tag in the profile with the Pentax tag to make ACR/LR "see" the Canon profile for the Pentax camera model.
You can make a copy of the 1Dx profile - call it something like "1Dx_for_Pentax645z.dcp" - then decompile it into XML using dcptool, edit the xml, and then recompile into a dcp. You will probably have to decompile a Pentax 645z dcp as well to find out the exact CameraIdentifier tag to use in the donor 1Dx file.
kirk