Is it possible to type in macron (e.g. "ā") without using U.S. Extended Keyboard?
This is just a option without changing the default Keyboard.

In OS X, this can be accomplished by holding down the A briefly and a pop-up will appear.



Just select by clicking or typing the corresponding number.  In this case 8

ā ← Like that

The repeat delay can be set to prevent eeeeeeeeee from happening too quickly.