louflan no unfortunately the current platform can not take in existing branding or layouts for context.
Since you are new to Relume, I would give it a go with just a simple description of the company and its goals, see what generates for a sitemap and layouts and you can quickly make edits to it.
We take away a lot of the tedious work of building out layouts and getting those layouts into Webflow already, so I think the time spent making manual edits is still far less than you'd have to do in any other platform.