This plugin lets you visualize your JOSM indoor data into indoor=. It adds a menu entry in the "More tools" entry and. On click it open the the current layer on indoor= as a preview.
The indoor= JOSM plugin is available via the Preferences -> Plugins panel. Follow the instructions and search for indoorequal.
Download the jar from the latest release: https://github.com/indoorequal/indoorequal-josm-plugin/releases/download/latest/indoorequal.jar and put it in the /plugins of your JOSM install: https://josm.openstreetmap.de/wiki/Help/Preferences#JOSMpreferencedatacachedirectories and enable the plugin via the Preferences -> Plugins panel.
GNU GPL 3