Hi @Jani73, you could try to use Microsoft Flow for this. In Flow you can find a template for adding newly Planner Tasks to an Excel Spreadsheet. So, I assume it should be possible to create a new entry in your SharePoint calendar, too.

Just open https://flow.microsoft.com, login with your account, create a new Flow and search for Planner. You should find the template and customize it to fit to your needs.