Is it possible to update ModelState.IsValid manually?

I would like to use the built-in validation features as far as possible. I would also like to use the same model for CRUD methods.

However, as a drop down list cannot be done using the standard pattern, I have to validate it manually. In the post back method, I would like to just validate the drop down list and add this result to ModelState so that I don't have to validate all the other parameters which are done with Data Annotation. Is it possible to achieve this?

I may be mistaken about the drop down list, but from what I read, the Html object name for a drop down list cannot be the same as the property in the Model in order for the selected value to be set correctly. Is it still possible to use Data Annotation with this workaround?

Thanks.