Appending characters to the end of each line in Emacs

You could use replace-regexp for this purpose, with the $ regexp metacharacter that matches end-of-line. Go to the start of the buffer, and then do M-x replace-regexp, and answer $ and (your text) to the two prompts.

Or, in emacs-speak, for your specific example of adding a:

M-< M-x replace-regexp RET $ RET a RET