combine 2 osm files

I have a osm file I downloaded to and customized for and area in louisville kentucky. I now need more of the city area but don’t want to redo what I allready created in the osm file. I want to add more of the city to the origional osm file that I have worked on.
I have been doing edits in josm.
How do I take 1 osm file with customiziations and add it to another section of the city to make 1 bigger file?

thanks for any help you can provide
Scott

I would expect that

• load your customized file in JOSM
• download any other area you need from the OSM server
• save the combined data

would work. But maybe I misunderstand your question

Mark both data-layer in JOSM → right click → merge