I don't know of any way to do this PERFECTLY.
One method you can use is to use UO Landscaper's multobmp program to convert it to a UO Landscaper compatible terrain.bmp and altitude.bmp.
The results will be less than perfect tho. None of the BMP-to-MUL programs have all of the transitions that would be needed to get a perfect copy. So I think you'll lose a lot of detail and a lot of the transitions will come out kind of screwed up.
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-= HellRazor =-
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