Set notation of a grammar
In contrast to popular thought, the description of a language doesn't have to be of the exact form
$$
L = \{a^i b^j c^k \mid \text{conditions on $i,j,k$}\}.
$$
A language is a set, and you can describe it in exactly the same variety of ways as you describe sets. For example, we can write
$$
L = \{a,b\}^* \setminus \{a^nb^n \mid n \geq 0\}.
$$
In your case, as rici mentions in the comments, you might as well just take a union with $\{abc\}$ to account for the additional case.