Sometimes lenses from one system can be adapted to another, but it depends on the mount’s flange focal distance. In general, you can only adapt lenses designed for systems with a longer flange distance onto bodies with a shorter one and still keep infinity focus.

For Pentax DSLRs, the practical answer is that adapting Canon or Nikon lenses is generally not a good plan, and in some cases may not be feasible while retaining infinity focus. Even when an adapter exists, you typically lose most or all automation: autofocus, electronic aperture control, automatic stop-down, and sometimes metering. That means manual focus, manual aperture setting, and often a darker viewfinder while composing.

For a beginner, this usually makes shooting much harder rather than cheaper. If you choose Pentax, it’s better to use native Pentax-mount lenses, including older manual-focus Pentax options such as Takumar lenses, which are more straightforward to use on that system.