How to set a member which will not roll up to its parent member

I understand that base members will be automatically roll up to parent level. For example:

Parent member (Revenue) (Calculated Result: 80)

• Child A (Revenue) (Inputted: 100)
• Child B  (Expense) (Inputted: 20)

I wonder to know is it possible to set a base member which will not roll up to parent level (let's call it a dummy member). For example:

Parent member (Revenue) (Calculated Result: 80)

• Child A (Revenue) (Inputted: 100)
• Child B  (Expense) (Inputted: 20)
• Child C (Dummy member) (Inputted: 60)

Many thanks