Hi,
1] The default behavior if you import line features is that CE creates (dynamic) streets.
2] You can only assign rules to shapes, not street graphs. CGA only works on shapes. So importing street center lines is basically a process to create shapes, for later use with CGA.
3] CityEngine 2013 will have a functionality to snap the street width to the neighboring footprints, so exactly the thing you need. But note that static shapes ( imported ones ) are not interacting with dynamic shapes ( streets, blocks, ..). So the area will not be filled, only the street will be made wider, but stay with mainly 'rectangular form'.
ok ?
m.