Source doesn't mean what it does to the layman for intl taxes.
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).
If Mr. X has no US-source income and earns income in a country with no totalization agreement with the US he can save the money he’s not paying for Social Security and Medicare in a savings account, IRA, etc. it’s not like the government-run programs are paying high interest.
