I'm from India, I bought retail copy of Witcher 3 and I would like to pre order Hearts of stone DLC. I can't purchase it as the price is in US dollars.Currency conversion is not possible here.

Can I download the DLC if my cousin in USA purchase the DLC through my account?

That sounds quite a strange problem, as we do have other Indian users here who have made successful purchases. Have you tried making a purchase from here and it failed? Assuming that your cousin's payment goes through successfully then, yes, you should be able to access the DLC in your account when it gets released (13 October).

May be it's problem with bank as some countries have different accounts for international transaction.
Btw ask your cousin to buy you season pass instead of single dlc.

I tried purchasing things in ebay which was in US dollars.. At that time I couldn't purchase since currency conversion is not possible here.

If any of your cousin is in russia then ask the russian one instead of american.
As prices in russia is much cheaper than in USA.
Try Visa card as it's most common debit card and if it fails too then simply contact your bank.
They will tell you what minimum amount you need for international transaction.

Did the retail copy come with GOG key so you have it on your GOG account? You can't purchase DLC for game you don't own on GOG.

Ya I'm thinking about that... Any chance of discount in season pass in future?

May be during sale