Steps:
1. Create object.
2. Add Rigidbody2D
3. Add Animation to this object
4. In Animation window press Add Curve. Add curve for Rigidbody2D mass
5. Select Rigidbody2D.mass curve and press Curves button
6. Drag curve down to negative values.

In curves window you can set negative values to the mass.
Negative mass will not be actualy set when animation is playing.