Figured out how to trigger an 'import' of existing vm via API, by passing the 'compute_resource_id' parameter and the 'uuid' parameter (set to the instanceuuid of the vm in vmware). Not really documented how to do this, but this commit gave me a hint:

https://github.com/theforeman/foreman/pull/4195/files

especially the changes in app/controllers/api/v2/hosts_controller.rb

So thats great, but when doing this and setting a hostgroup which is connected to a compute profile, the compute profile settings are enforced on the VM in vmware. So when I import a VM into Foreman via API, Foreman changes cpu/memory/disk settings for the vm in VMware. That seems wrong to me.

If i do an import via GUI Foreman does not actively enforce the compute profile settings.