If you already have a working Puppet infrastructure, I’d rather suggest you deploy a dedicated Foreman machine, and then integrate the existing Puppet Server using a Foreman Proxy. That way the changes needed to be done to the existing Puppet Server are much smaller and you limit the risks.
If you really want to install Foreman on the existing system, we’d need to tell the Installer to actually install Foreman, but leave Puppet untouched. This should be possible, but at the same time this is most probably an untested path.
Do you have a copy of your prod setup to play with?