I have added data (e.g. name keys) to my OSM PostgreSql database. How can I update my database without deleting or overwriting this data? In other words, how can I merge newer OSM data to my existing database without losing my additional data?
In the OpenStreetMap environment so-called "diff-files" are used to update your PostgreSQL/PostGIS OSM-DB. This can be done based on minutely, hourly or daily diffs. There are several tools to do this, I recommend to use osmosis. This OSM wiki page gives a comprehensive overview.
In the German OSM forum there is quite a good thread on this topic. As already mentioned by @Toby Speight in the comments you should keep your updates seperated from the original OSM data so they wont be messed up.