The multiverse is infinite. Even at an infinite and exponential rate of conquest, it would be impossible to reach the ends of the multiverse for there are none.

However, one could still because a powerful motherfucker. The Combine are smart. They took over weaker civilizations and dimensions, and then used the collection of weaker ones to take out ones that are a bit stronger. Rinse, repeat. Eventually, they can work their way up to Sues. At that point, there is nothing that could stop them. Even another Sue-verse could be defeated, since one Sue-verse is being properly coordinated to defeat them. Also, numbers of other goons. NOTHING CAN STOP THE YORPS

Have a funny, tangently-related video:

Oh, a multi-verse picture from Rush that's actually funny. I'm impressed.
Here, take my like.