My W-2 lists nothing in boxes 18-20, though local tax is collected in my locality. Should I manually add my wages in box 18 & locality in box 20 to determine what I owe?

No, 

if your employer did not withhold any local tax you will need to file a city return. 

You would allocate part of your income to that locality on that return, you do not alter your W-2.

TurboTax does not support city returns for the state of Pennsylvania. 

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