If I'm reading correctly, you want to offset a line the exact same increment multiple times, without having to select each new offset line, as you would in the normal offset command?
If so, the "Extended Offset" lisp in the Express menu is what you would want...
You specify the distance, select the line to offset (or pline in your case), then "M" for multiple, and select the side to offset to, and keep clicking for each one...They will offset the specified distance from the previously offset one....
Hope I'm on the right track...if not, that might help someone else...