You’re right, doesn’t happen on a clean install.
Don’t know which is the best option (the one with lesser hassle for both you, as maintainer, and users). stephdl or dev_team might be able to give some advice.
Hosting the package on your repo you’ll have more control on updates and possible troubles at the cost of extra bandwidth usage. Using nethserver-onlyoffice to download external package (i.e. as some flash player packages did IIRC) you have control on the order of installation and signaling events without affecting your bandwidth, but don’t know if it’s considered a good practice or accepted by the terms of use of external package.