Another possibility would be to write some sort of connector between the two APIs. Like a cronjob that runs every minute or so that asks both the Foreman-API and the Checkmk-Web-API for their respective list of hosts. Then it calculates the differences between these two lists (e.g. new_hosts = F-List - C-List and vanished_hosts = C-List - F-List) and eventually calls the Checkmk-API to create the missing and delete the vanished hosts.
This is basically the same thing @andreas-doehler suggested. It would mimick the DCD a bit and (AFAIK) would run with the raw edition as well, which might be an issue.