I've designed a site using Museand would like to have fluid pages sizes. Is it possible to import the HTML into DW and convert the page layout? Or do I have to start from scratch?
You'll need to start over with a Layout that supports CSS Media Queries: one style sheet for each target device width (Mobile, Tablet & Desktop).
Introduction to CSS Media Queries
DW CS6 Fluid Grid Layouts (6 min video)
DW CS6 Fluid Grid layouts (17 min video)
Creative Cloud 12.2 Fluid Grid Layouts
Step-by-Step tutorial -- Building Fluid Grid Layouts in DW CS6