The express tool to which you refer only converts HP-GL format plot files, the really old kind generally for pen plotters. Most drivers in the last 15 years or so that write HP-GL plot files write the
HP-GL/2 plot file format.
I wrote up something on this a few years ago, along with some suggested programs to do what you want.
http://cadpanacea.com/node/17Just remember, a plot file has lost the accuracy of the DWG, along with things like layers, blocks, styles, etc.
So converting back to DWG only gives you a rough estimate of the original drawing.