If Southasian-ness is a sensibility, then anyone can be a Southasian. The requirement is having respect for the history of the Subcontinent and empathy for its people. You can be a native or a non-native. Of course it helps to be born in any one of the countries of the region, but you can also be a naturalised Southasian due to the empathy you develop for this region and its populace.
In contrast, people who are natives could also not be Southasians, particularly out of lack of concern for others in the region. The growing nationalist insularity in Southasia, for example, has left some natives not knowing their neighbours and others harbouring exaggerated animosities – neighbourly xenophobia. Whereas a non-native Southasian tends to come more free of prejudices. For starters, he is less likely to say India-Occupied Kashmir and Pakistani-Occupied Kashmir, and more willing to try out the less problematic India-Administered Kashmir and Pakistan-Administered Kashmir. All Southasia, including Indians and Pakistanis, need to ascend from IOK/POK to IAK/PAK.
This definition of 'Southasian' involves the quality of empathy, more than any other value or marker. Identity is multitudinous for someone from the Subcontinent, in terms of locational linearity from the village mohalla to city, district, state/province to national. No problem there. But to jump from the place-of-origin identity to become a citizen of the region of Southasia, you need the quality of understanding the point of view of the other, competing person or community. This means not only a Pakistani seeking to see the world through the lens of an Indian, but anyone of us being considerate across the divides of faith, caste, ethnicity, region or language.
It is especially difficult to be empathetic across national boundaries. After all, most of our nation states are no more than six decades old and the nationalist identity is still being consolidated under the leadership of the capital-based elites. That is why we so easily allow barbed-wire fences to be built along the frontiers, and why ultra-nationalism is such an infection, the tool of demagogues. Ultra-nationalism lurks just under the epidermis of even the most magnanimous civil-society leaders, because human evolution has provided us all with the capacity to hate and it is safest to vent against the foreigner. The neighbour, the neighbouring country, is simply the easiest to loathe. Those who are not native, on the other hand, are outside the ambit of this dangerous loathing.