Have you tried doing this in Wilbur with the source material that you have?

Using the image posted above, the histogram looks like this: nice flat spikes at each filled contour:

esqof_h1.gif

Select the land, add noise (about 10% of your maximum altitude), fill basins, incise flow (2, 0.3, 0.7, 0, 0.5, 0) will get you this image:
esqof.jpg

And its histogram looks like this:
esqof_h2.gif
Notice that the spikes have broadened considerably and to the left of each original spike (that is, the land has been removed and generally not added much). The broadening is partly due to the noise (the histogram for the noise step will look like slightly widened spikes), and the leftward tail on the original plateaus is due to the incise flow operation being a remove-only operation.

This operation does change your original contours somewhat. Anything other than a simple interpolation will do so, including your stated goal to "add the final details in wilbur".

This kind of processing in Wilbur does have another significant issue: Wilbur can only process what it has in memory. On the above processed image, you'll note that river systems along the bottom edge of the map are running completely the wrong way (straight at the edge of the map). For best results in Wilbur, the coastlines need to be visible all around in order to prevent the weird edge effects. Or you can make the terrain a little larger and draw a tall wall on the waste space to keep everyone heading in the same general direction, but then you won't get rivers heading into the map from off-image.