Unfortunately no you can’t use your existing well cap with the Simple Pump well pump system.
Simple Pump well caps are specifically designed to position the Simple Pump well pump system in your well. Existing well caps typically contain ports on the top surface of the well cap that interfere in the positioning of machining a port for the Simple Pump.
If you have a pitless well cap, it is unlikely to have a flat surface to mount a Simple Pump. Commonly, existing pitless well caps were not designed to support a Simple Pump installation.
Simple Pump provides well caps for pitless and non-pitless wells. We also have our own adaptors for wide dug or bored wells with covers.