Le Châtelier's principle states that when a chemical system at equilibrium is disturbed, the system will shift in the direction that opposes the change in order to re-establish equilibrium.

In the case of the addition of N2O4 to the system, the equilibrium will shift towards the formation of more NO2. This is because the addition of N2O4 increases the concentration of N2O4 in the system, which means that there is more N2O4 available to react with NO to form NO2.

The reaction can be represented by the following equation:

$$2NO_2(g) ⇌ N_2O_4(g)$$

The addition of N2O4 will cause the equilibrium to shift to the left, resulting in the formation of more NO2 and a decrease in the concentration of N2O4.