Hi kin.gyo,
You can create a *.dfs2 file from a *.dfsu file by creating a new "Grid series (.dfs3, .dfs2)" file. When creating this file, you can then select "from Dfsu file": this will allow you to convert your dfsu file into a dfs2 file. You can then convert this dfs2 file to ASCII by use of the following toolobox: MIKE Zero Toolbox \ GIS \ Mike2grd.
If you want to export only a part of the whole domain from your dfsu file: you can either crop the generated dfs2 file (in Tools \ Crop) or crop the dfsu file using the Data Extraction FM tool (New \ File \ Data Extraction FM).
I guess this answers your questions.
Mathieu