Buck Thorn's answer already addresses the conceptual ideas, and Sam202's how to solve it brute force. In this answer, I will first show an alternate way of approaching this using the conservation of mass. Second, I will show an easier way of filling the ICE table, and use the ICE table to illustrate why a set of related starting conditions (the dollars/pounds analogy in Buck Thorn's answer) lead to the same equilibrium.
Conservation of mass
We calculate the amount of nitrogen atoms in the system. We start out with 0.6 mole (from 0.3 mole of dinitrogen tetroxide) and add another 0.2 mole (from 0.2 mole of nitrogen dioxide) for a total of 0.8 mole. If at equilibrium, 0.6 mole are in the form of nitrogen dioxide, the remaining 0.2 mole nitrogen atoms have to be in the form of dinitrogen tetroxide, showing that there are 0.1 mole of dinitrogen tetroxide at equilibrium.
ICE table
First, the ICE table correctly given by the OP:
$$
\begin{array}{lccc}
\ce{&N2O4(g) &<=> & 2NO2(g)} \\
\text{I} & 0.3-x && 2x+0.2\\
\text{C} & +y && -2y \\
\text{E} & 0.3 - x + y && 0.6\\
\end{array}
$$
And this equation is super hard to solve.
We can replace the $-2y$ with an expression containing $x$ because the value in rows "I" and "C" have to add up to the value in row "E".
$$
\begin{array}{lccc}
\ce{&N2O4(g) &<=> & 2NO2(g)} \\
\text{I} & 0.3-x && 2x+0.2\\
\text{C} & +y && 0.4 - 2x \\
\text{E} & 0.3 - x + y && 0.6\\
\end{array}
$$
Then, we can replace $+y$ (via the stoichiometry) and $0.3 - x + y$ (via addition in the column). Or you solve $-2y = 0.4 - 2x$ for $y$, and substitute into the expressions still containing $y$:
$$
\begin{array}{lccc}
\ce{&N2O4(g) &<=> & 2NO2(g)} \\
\text{I} & 0.3-x && 2x+0.2\\
\text{C} & x - 0.2 && 0.4 - 2x \\
\text{E} & 0.1 && 0.6\\
\end{array}
$$
As you can see, the $x$ is still in the table, but canceled out for the equilibrium concentrations, so we don't need to know it to calculate the equilibrium constant.
[from OP's comments to Buck Thorn's answer] How can I be sure that adding the 0.2 moles of NO2 later is going to produce the exact same result as adding it in initially?
There is an entire set of starting conditions that gives the same equilibrium. In equilibrium problems, there is no "order of addition" effect. You can add one substance first, then the other, or back and forth. As for the ICE table, I can take the equilibrium concentrations and show you the entire set of starting conditions that yield the same equilibrium (this ICE table is unusual in that we are calculating initial conditions from known equilibrium conditions):
$$
\begin{array}{lccc}
\ce{&N2O4(g) &<=> & 2NO2(g)} \\
\text{I} & 0.1 + z && 0.6 - 2 z\\
\text{C} & -z && 2z \\
\text{E} & 0.1 && 0.6\\
\end{array}
$$
As long as I don't get negative values for the initial concentrations, I get the same equilibrium. So $z$ can go from 0.3 to -0.1. Here are a couple of examples:
\begin{array}{cccc}
z &\ce{N2O4(g) &<=> & 2NO2(g)} \\
0.3 & 0.4 && 0\\
0.2 & 0.3 && 0.2 \\
0.1 & 0.2 && 0.4\\
0 & 0.1 && 0.6\\
-0.1 & 0 && 0.8\\
\end{array}
I can take the initial conditions in any row of the table, and will get the same equilibrium (one is already at equilibrium, with $z = 0$).
I should say that the initial conditions are a thought experiment anyway because as soon as you add all the reactants or all of the products, the reaction starts, so the "initial" concentrations are never present in reality.