You can add or remove a SLOG device at any time.
However, warning, warning, be careful, it has happened to people that they accidentally add a device to their pool as a new vdev instead of as a SLOG. This is kinda catastrophic, so be sure you are reading the manual, maybe see if you can find a recent YouTube video, etc., to make sure you understand the correct steps to add the SLOG device as a SLOG device.