How can I type Lithuanian stress characters (e.g., ą́, ė̃, m̃)?
For info on how to add combining diacritics to any character with ABC Extended, see this page
http://sites.psu.edu/symbolcodes/mac/codemacext/
For example, m̃ is made by typing m and then option-shift n. ė̃ is made by typing option w, e, then option-shift n.
The keystrokes for adding acute ́ and grave ̀ are option-shift e and option-shift `.
You might want to create a customized version of the Apple Lithuanian keyboard using Ukelele to adding keys for the combining diacritics used to indicate stress. Here is one you can try.