How to add conditionals for IE browsers between certain versions?

Could you not just pass the different string?

wp_script_add_data( 'foo', 'conditional', '(gt IE 6)&(lt IE 9)' );

Which by the way is probably the same as:

wp_script_add_data( 'foo', 'conditional', '(IE 7)|(IE 8)' );

IE conditionals are pretty unreliable so need to be tested.