P.S, I know this is pretty late in the game, but could help other :)
You could also do the following
1) Go to your sharepoint admin site (https://<your_organization_name>-admin.sharepoint.com
2) Click "Apps" on the left hand navigation pane
3) Add App catalog
4) Upload your app to this app catalog and this way it would be available for all of your sites to add.