How to force 0-255 level mapping in Xpress 4.0 as in CCE

I like the TMPGEnc encoding better in some ways, and want to give it a
fair trail. But when i use Xpress 4.0 to encode my captured RGB32, i
fail to get the same levelmapping that are possible in CCE with a
lumens setting of 0-255, instead of 16-255 so i can get the desired
dynamic range.

Without this levelmapping Xpress 4.0 looks washed out, as CCE whould do
if luminace 16-255 level mapping was used.

Is it somehow possible to force/change TMPGEnc Xpress 4.0 to use 0-255
level mapping as CCE does, and how is it done?