sure, the first step would be to query all the features you want to update - for example this query https://overpass-turbo.eu/s/1tZI finds all ways (lines & areas) with "name"="North Island Main Trunk" but no "railway:ctc" tag

it's generally a good idea to do small sections at a time - you can limit your query to an area with the 🖼️ button on the map.

then you can click "export" to load the data into JOSM or the Level0 editor. I personally don't use JOSM but assume it has a method to select all features in a layer