create a UserDefinedCommand of MultiScript Type
insert this script code
@var $names = GetSelectedFileNames();
@var $count = arrayCount($names)
for($idx = 0; $idx < $count; $idx++)
{
$names[$idx] = StrReplace($names[$idx], " ", "%20");
}
@var $text = ArrayToString($names, "\r\n", 0 );
SetClipboardText($text);
Save it, and assign a hotkey or add it to the menu.