I already have custom HTML, css how can I import that webflow?

hi @parul and welcome before you post any request please use input search field here on forum or your favourite browser to look if there was any request to topic posted and read results if there is solution posted.

Anyway at his moment is not possible

but one way you can use is embed element.

in meantime you can vote for it

https://wishlist.webflow.com/ideas/DESIGNER-I-12

but if you have design done it will be most probably easy to recreate it in WF