Hello Vasanth,
if you want to use a dropdown, you will have to manage your CUF position directly inside the Web Dynpro Component and to follow same procress as for Purchasing Group.
If you do not want to enhance the WDC, you can link a search-help to the data element you used for your CUF and filter out values through the search-help exit, based on your business rule.
Regards.
Laurent.