Hi @amosbird
Welcome to the community forum 
This is not possible via CLI at this time, scenarios can only be configured via a code/config file.
Can add the feature request in the k6 repo for the team to consider in the future? What we would be looking at there is adding support for Scenarios in CLI, and you can also describe you particular case, you need to set rate in constant-arrival-rate. I can open it myself if you prefer, just let me know.
Cheers!