So far, I found only the following work-around, though unsophisticated...

1) Make a new worksheet which has the same number of columns to the number of levels in your contour. Fill 0's at the only first row of every column. Put the level values to their column labels.
2) Make a line graph. Remove the axes and axis labels from the graph. Notice that only legend is visible. In the Plot Details dialog, set the graph group as "Independent".
3) Merge this line graph with your contour map (by Edit:Merge All Graph Windows - set both number of rows and columns 1). You get the legend in your contour.

Now, you can adjust the line color by double clicking the line sample in the legend (when tthe legend frame is not selected). If you want to change the legend title, change the column label in the worksheet.

Please try this :-)

I found that this has been already proposed to the development group, though I don't know when it can be implemented. I increased the number of requests in the database. Thank you for your suggestion.