You can easily copy and then paste a block in Eeshema within hierarchical sheets of the same project.
If you want to copy a block from another project into a new project, you can temporarily include the already existing schematic as a sheet in your new schematic.
Adding already existing sheets into a hierarchical design is (briefly) handled in the mini tutorial I just started for multiple PCB’s in a project:
https://forum.kicad.info/t/multi-pcb-project-tutorial/17657