I do it manually, too. I have not found a way to automate it.
Make sure the "build.properties" file has all the right dependencies ticked.
Select the project(s) in Eclipse and choose "Export -> Deployable plug-ins and fragments" and export the jar(s) or folder(s) into a directory.
BTW - Archi plug-ins in the "*.archiplugin" format are the exported jar(s) zipped into a zip file (the extension is renamed) together with the magic "archi-plugin" marker file so Archi knows that it's an Archi plug-in that can be unpacked and copied to the "dropins" folder (rather than some random *.jar file). In fact, you could just manually copy the *.jar file(s) to the "dropins" folder.