Part of the Marian Hegemony... ;D
Heh. Unlikely.
Seriously, though, the Marians had ample opportunity to do so after the Jihad and during the Republic era but didn't. Post-Jihad, Niops would have been down to less than two companies of 'Mech and vehicles, a squadron of ASF, and maybe a battalion of infantry (ie. 10% of their pre-Jihad strength), appear to have lost production of one version of the Nighthawk to the Marians (the XXI is not on the master unit list for the Marians during the Jihad, but the XXII is), have lost most of their military production, and had their capitol nuked.
Given all that, why didn't the Marians conquer Niops in the near five decades between the end of the Jihad and kickoff of the Dark Age?