I hope you all vote on DeveExrpess Issue that will allow us to validate runtime extended existent types. But even if you did not there is an alternative way that my friend Emilio shared with us.
I want to add runtime members for my existent Customer class for which records already exist in the DB and also use the validation system
You can create a new dynamic type that inherit your customer class decorate it with the MapInheritance attribute and set the MergedObjectType same as BaseType
Download working sample app with source code here