Can this function be added somehow manually in this program? Maybe like style?
No, there´s a widget from muse-themes.com that comes with a price, I guess but it´s just a style and no real lazy loading, though
No way. Muse does not support SRCSET and never will because Muse is discontinued. This is another good reason to switch from Muse to modern software.
This Lazy Load extension from DMX Zone works in Dreamweaver which does support SRCSET.