No, I just want to register a new model in the storage.

Is the model file is already uploaded, you can register it without a Task:
InputModel.import_model(...)https://github.com/allegroai/clearml/blob/b3a2b3425c5098ebfc0598c9dfb3e670d4a87706/clearml/model.py#L521

I need to create a separate task for this right?

If you want the model to be uploaded, then yes you have to create a Task.