Hi Bin,
You can not use KUT field to define field XBO.
There are some basic you should understand:
1.For defining field in BO you should use 'Data Type'
2. KUT field(XMLB) has already created why you want to create in BO?? If you want to use in logic(ABSL) you can create a reference of this field in SDK, Please read the chapter "Create References to Customer-Specific Fields" in SDK document https://help.sap.com/sdk
IF you want to create a code list type in SDK then in SDK you can create CodeList
Read Chapter 8.6 Code List Data Type, in SDK document.
Regards
Sunil