Use the design system you already have.
Extract design tokens from websites and use them directly in Figma.
Available for free on the Figma community:
Get Superposition for Figma
Add colors to local styles or use them as fills.
Transform typography into what's being used on the site.
Apply radius to rectangles and vectors.
Use the same shadows as your site, even layered ones.
Add images to your canvas or use them as background.