While not impossible, it would be difficult to use a template from webflow.com within the LP editor. First reason being, is Marketo LP's are not responsive. Marketo's Landing Page Editor uses absolute positioning on elements that are inserted onto the page.
Also, Marketo LP's have to have specific sections in order for it to work properly with the editor. so you would have to semi-rebuild the template by using those sections. I would create a new template, and then familiarise yourself with the structure of the HTML/CSS. I think that will give you a good idea on where to start if you want to convert a webflow template to a Marketo Template.