Try this:
View Form 8962 in Forms Mode in TurboTax desktop.
Right click on Line 2a (Modified AGI).
Select "Data Source"
Select the link from the pop-up box for "Form 1040 Total Social Sec" Benefits
You can "override" the lines on the Social Sec Benefits worksheet, as needed, to add a (nontaxable) dollar amount that will correspond to your nontaxable foreign pension income (excluded by tax treaty). The amount you enter here will "flow" back onto the Modified AGI Line 2a on Form 8962, and should not affect the rest of your tax return.
