You can add etc. to your dictionary, but I don't think it's going to help you much ...
It's relatively easy to add words to your User Dictionary. Just go to to your Edit>Spelling>User Dictionary menu command to open/create a user dictionary for your language, then add the word. Double- and triple-check, though, that the word you enter is absolutely spelled correctly. Or you'll be misspelling it the same way forevermore.
But the problem is that it isn't alerting to the word etc. It's alerting to the period character itself. Like in the example shown at left, the word menu is causing an alert, even though the word Place is in fact part of the English: USA H&J dictionary as shown below the dialog box, because the last period in the ellipse is what's causing the false alert on the spell-checker.
It sucks, but unfortunately that's just the way it is. I just mutter and cuss a little under my breath and deal with it.
Sorry I don't have a better answer for you, but it's the true one.
Randy