AutoCad :: Merging Polylines Into A Polygon?

What I want to do basically is merge a couple of polylines (which have different widths and may overlap each other) together into a single 'shape'. I am calling it shape, but I think the correct term would be polygon.

The end result should ("from the outside") look the same as before merging, the only difference being that the polylines are now merged into one shape. Important note: it cannot be a polyline with segments of different width, I know AutoCAD supports this, but the drawing will be exported to a format which does not seem to support this (and the result then is a line with a single width - wrong).

Here's a mockup of what I want to achieve:

On the left are three polylines (PL1, 2, 3), where PL2 has a different width and overlaps the other two polylines. On the right is the desired result: a single polygon (NOT a polyline with variable width) in the same shape as the polylines.