Some low-end Shimano dérailleurs have an integral "claw" adapter to essentially make their own hanger, and you can buy the adapter separately. But there's a lot more to that change than you might think.
- shifters
- cable and housing
- cable and housing support points on the frame
- how to mount the cassette/freewheel to the hub
- need to replace wheel hub? i.e. complete wheel rebuild or replace
- width of hub/cassette/freewheel compared to the frame dropout spacing
- resize chain
- learning to shift
A bike designed as a single-speed will be a PITA to convert. A bike designed as a multi-speed and converted to single-speed will already have some of the features you need.