You can use a domain you already own for your website. Using the domain costs the same price as booking a new one (1.99€/month, 1 domain is included in the Premium package and 3 domains in the Pro package).
If you do not have any SiteW website yet, signup and choose a temporary .sitew.com website address first.
Then, go to the My websites page and click on the Settings button, followed by Domain names and emails.
Finally, click on the link I already own a domain and would like to use it for my website then validate by clicking on Book selected names.
Once your order has been validated, you will be given 2 possibilities: