hello,
ya by applying region command on a box ,cube or rectangle made by using line command we can change it to pline . also by using pedit command. type pedit then select a line then
Do you want to turn it into one? <Y> Y
Enter an option [Close/Join/Width/Edit vertex/Fit/Spline/Decurve/Ltype gen/Undo]:choose join and then select lines one by one andf ure lines will convert to pline.but it is only possible when ure drawing is closed like cube ,rectangle etc