When the 'uuid' and 'compute_resource_id' parameters are set when POSTing to /api/hosts, Foreman imports the VM from a compute resource [1]. However, the UUID parameter is not present in the param_group 'host' [2] and is not documented in the API reference [3].
Overall, this feature seems to be pretty undocumented.

1: https://github.com/maccelf/foreman/blob/b9986c9648b55c0d8fae4ab5ff5845763ddbea1f/app/controllers/api/v2/hosts_controller.rb#L136
2: https://github.com/maccelf/foreman/blob/b9986c9648b55c0d8fae4ab5ff5845763ddbea1f/app/controllers/api/v2/hosts_controller.rb#L72
3: https://apidocs.theforeman.org/foreman/latest/apidoc/v2/hosts/create.html