Technically you can do this. Commercially there are a few comments:
1. Licenses are assigned to a tenant so you cannot pool licenses between tenants, you can move licenses across but tenant a has x and tenant b has y licenses at any point in time.
2. It used to be the case that a separate tenant would require a separate HDA (contract) number, this should be discussed with your account manager or partner.
1. Create a new DNS name for the instance department.company.com
2. You can run the system config wizard and add a new database using it with your new DNS name (you only need one config server so just a new application db required) Doing it this method will require a restart of services and take down your prod system. It can be done without shutting down services but you need a blank database to restore manually.
3. Staging will sort itself out from memory.
I would discuss and sort the commercial aspects before looking at the technical.
It's more at POC stage of the process, but we would get few additional licenses for this specific tenant, or send our "over-licensing" licenses to this tenant.
For the technical aspect, then the wizard will do everything on its own. I was scared it might "replace" the existing IIS application.
I'll give it a try and let you know if it worked
Thanks for your help
Do not do it in production. Do it in staging to test.
Sure, I'll try to keep my head on my shoulders until 2018 ;-)
Thanks anyway for the advice!