If I understand what you are going for, you want to apply Susy's
'+container' mixin to the various internal containers that are a fixed
width. You don't apply any Susy mixins to the outer containers, and
they will be 100% width by default. If all you need is one repeating
background image, you just put it on the body (as G. D. recommended).
body
+susy
#outer
:background...
#header
+container
#content
+container
etc. You can have as many 'containers' as you need.
To use Susy for a fixed-width design, just set your !grid_units (in
_base.html) to 'px' and define column and gutter widths as px widths.
Susy still by default will not let the +container elements be larger
than the viewport. I recommend leaving it that way, but you can
override it by setting max-width on those elements to anything else
you like.
Hope that all makes sense.
Cheers,
Eric