Will my Medicare Part D plan reimburse me if I purchase a formulary medication outside the United States?

Only in very limited situations. In 2016, the Centers for Medicare and Medicaid Services (CMS) updated their Prescription Drug Manual and added some guidance for the exceptional circumstances when formulary medications purchased from foreign sources can be covered by your Medicare Part D plan (Rev. 18, Issued: 01-15-16 is still the most current version of this manual).

In Section 10.7 - "Drugs Purchased in Another Country", CMS notes that:

"Part D sponsors must exclude Part D drugs from qualified prescription drug coverage if they are not sold in the United States. In addition, Part D sponsors may only pay for drugs that satisfy the definition of Part D drug. In general, such definition requires FDA approval for sale in the United States. Therefore, even if the manufacturer has FDA approval for a drug, the version produced for foreign markets usually does not meet all of the requirements of the U.S. approval, and thus it is considered to be unapproved.

[However, i]n the event of a drug shortage, in order to ensure access to critically needed drugs in conjunction with the actions taken by the FDA, CMS will accept PDE [prescription drug event] submissions with national drug codes [NDCs] for foreign versions of Part D drugs, imported under the FDA’s exercise of its enforcement discretion.

PDE [prescription drug event] allowances will be limited to the specific drug product(s), conditions, and the duration of the shortage as specified by the FDA."

[emphasis and links added]

Source:
Medicare Prescription Drug Benefit Manual
Chapter 6 – Part D Drugs and Formulary Requirements
(Rev. 18, Issued: 01-15-16, Effective: 01-15-16; Implementation: 01-15-16)