Hi,
load merge would be used to load data into CDB. "Sync-from" is an RPC/Action and not something you store in CDB. So, to you question, the answer would be no.
Moreover, your idea does not sound like a safe operational process. What I would suggest you is to set a service state leaf and some service level action. In that sense, the service is first committed but in a state that does not push config into the devices. When the device is ready, the operator would call the pre-built action via GUI/CLI/API. The action code would take care of sync-from an change the service status.
Roque