You cant upload data from two different datasets reliably into the same planet_osm tables because there may be conflicts. The appropriate way to do this is to merge your north american data with your european data before importing it. Suitable tools for the latter operation are osmosis and osmconvert.

If you dont want to do this, the alternatives are:

  • Import Europe into differently named tables (eg eu_osm_). It is possible for a knowledgeable DBA to then alter names within the database and build views so that the two datasets are both queried.
  • Create a separate database on Postgis for Europe. Same problem as above, but no realistic possibility of merging data through views.
  • Use tricks within osmosis to convert current Europe data to look like an OSC update on North America. No idea if this would work. Requires deep knoweldge of OSM & osmosis.

Basically the best approach is to re-import your database with merged North America & Europe. Note adding Europe will increase DB size by as much as 200% and may impact import processes.