I grabbed original Z-Terrain (2048x2048x384) and made this:
After importing result was... Unpleasant... 7000+ sectors turned into 14000+...
What I did wrong here?(
UPD: I just solved it. Seems like exporting from MM3D and then importing into UDB takes almost forever time with strange results. Then I found that I can just import exported from UDB *.obj model just with something like "Scale = 8.0". Seems like for my goal there was enough just UDB...
You can scale the terrain z axis by picking vertex editing mode in doom builder then selecting vertices that compose terrain you desire to scale and click right mouse button on one of those. Then enter desired multiplier value in "Absolute floor height".