Mr X is a US citizen who lives outside the US, files and pays US taxes (with deductions taken for locally paid taxes). Mr X wants to also pay into social security and medicare (and whatever other right accrual systems) so that, if they decide to go back to the US so that -- when they retire -- they are entitled to social security benefits and medicare coverage.

The question is: How does Mr X technically pay into these systems, without having any US based income. He knows how to pay income tax (and does), but can't figure out a way to pay SS/Medicare (and doesn't).