Hi Payal,
As I understand it, this isn't possible using processing rules :-(
The 'countries' data is created from the IP address during processing that Adobe does post processing rules. So you can't using processing rules to map it to somewhere else because it doesn't exist at point that processing rules can manipulate the data.
If this is really important to you, it might be possible for Adobe Engineering to build a VISTA rule to achieve this for you (however, there would definitely be a cost associated with this and there are no guarantees it is even possible) - speak to your Adobe Engagement Manager if you want to try and purse this route.
One thing I don't understand is why you want to get the 'out of the box' 'countries' data into a prop or eVar? Why is it not possible to achieve what you want to using the 'out of the box' variable? - if you describe what you are trying to achieve someone may be able to help you avoid the need to get this into a prop/eVar.
Kind regards,
Andrew