JOSM is my favorite editor! It has so many functions and plugins that almost every task you can think of is covered (besides splitting multipolygons, but work has started to remedy that).

Today I learned how to add new nature reservations from a geojson layer by merging selections from it into my osm data layer.

1. Select the new element in the json layer
2. press ctrl+shift+m or edit->merge selection
3. switch layers and edit the tags
4. upload

Easy peasy!