TBH, I never actually tried to integrate an external Puppet server with Foreman.
About a year ago, I have wrote down my thoughts about how that might be achieved for a Puppet enterprise system here, but there was no further response whether that worked.
The easiest way would probably be to set your Puppetserver up as smartproxy, but that will configure Foreman as ENC as well (which I do not know if you want).
There is an installer option (–puppet-server-external-nodes) that you could use to point to a different ENC script, but I can not guarantee that this is the only option you will need to set depending on your setup.
Regards