Shashank,
Narendra's suggestion is a good one!
Probably with standard SAP you can satisfy your requirement, by following Narendra's suggestion.
To elaborate the idea -
By text determination, you can have a popup during the creation of a sales order.
This popup shall "get populated" only for the select customers.
In this popup you can have a text e.g. "Maintain manual condition type ZXXX"
Probably a demerit is that after reading the popup, still the user can choose not to input the value for this manual condition type! And there is no place in the system to check point.