OData service use filter for fields not present in entityset

I have an OData service created through SEGW which has an entity set imported from DDIC source, I want to use $filter to pass two fields to use in the select statement where clause. I don't need those two fields anywhere else and these two fields are not there in the entity set.

How can I use $filter for fields that are not a part of the entity set?

If I pass these two fields in $filter, it gives me an error "Property not found in entityset".

Is there a way to fix this or any other method on passing parameters to the getEntitySet method?