If you mean you have a table with 100 fields, but you only need to map an Entity with only fewer fields. then you can use the designer to manually seect/deselect the fields you want to be mapped.
If you mean you have a table with 100 fields, and you need to have an Entity mapped to all those fields, but in another situation you need a smaller entity (mapped to a fewer fields), then you may use the Designer to map another entity to the same table and manulally select/deselect the fields you want.