Going from a binary RPM to an OBS signed rpm?

I have a RPM that I'm currently building with FPM, fpm --verbose \ -s dir \ -t rpm \ --directories /opt/myprod --name myprod --rpm-summary "My Prod" --version $(git describe --abbrev=0 --tags) --rpm-sign --rpm-digest sha512 ./myprod=/opt/ This essentially creates an rpm **from a directory** that does **exactly** what I want. It works perfectly. But, my company has their own signing key that can not be shared on an [Open Build Service (OBS)](https://openbuildservice.org/) server. I don't *need* to use FPM at all if OBS can do everything. What I do need is some *understanding* of how I go from the directory ./myprod to an RPM which installs it under /opt/myprod. I'm new to RPM packaging. I assume I could take this directory and .tar.gz it and submit it to to OBS to be packaged and signed. I assume I need to somehow specify this with a spec file? What is the most simple way to accomplish this. Ideally, I wouldn't have to manually generate a spec file. Is any of this provided by OBS?