You can call yourselves Moahir there is nothing wrong with it, but I expect no one will ever blame the "sons of the soils" for imposing this identity on your folk. You can also call yourselves Karachites but it would simply mean a person who lives in Karachi, therefore the people of other non-Urdu speaking communities living in Karachi are also Karachites. Urduwan sounds very un-urdu btw, and I am sure if I ever call any Mohajir an Urduwan in Karachi he/she will definitely think who the fok is that animal, never heard before - and it will also be a big injustice to the Urdu lovers from Punjab, those guys have already assured themselves that Urdu is as theirs as Lassi. May I suggest something that sounds very catchy and very Urduish? What about "Urdu zoban bolne wale Mohajir"?