You can easily customise the double click action in recent AutoCAD versions to convert a line to polyline. Here are the steps to follow.
Type CUI in the command prompt to open CUI dialog box (see attached image). Goto 'Command List' section and click on new button to create custom command.
In the properties section, enter name as 'LineToPline' (or whatever you like) and in the macro section type in
orCode:^C^C_pedit;;;^C (Don't miss the last three semicolumns).
Now goto 'Double Click Actions' and locate the 'Line' action (See the picture).Code:^C^C$M=$(if,$(=,$(getvar,peditaccept),1),_pedit;,_pedit;;)
Simply drag the 'LineToPline' command from the 'Command List' section and drop it in the 'Line' action.
That's it. Now click on 'Apply' button and close CUI window.
To apply from a Custom Button
Create a custom button with the above macro. Preselect the line and click on the button.That's all you need to do. This method can be implemented in the older versions very easily.
Note: The last ^C in the first macro will overcome the problem of one extra semicolumn when the PEDITACCEPT system variable is set to 1.