I have never heard anybody in England, and certainly not in America, ever say "I agree with bells on." The Word Detective bit cited by Nick is just about right. There's also the phrase "with all the bells and whistles", meaning "with all the best gadgety features". And of course there's the famous rhyme,
Ride a cock horse to Banbury Cross,
To see a fine lady upon a white horse.
With bells on her fingers and rings on her toes,
She shall have music wherever she goes.
(Banbury is between Stratford & Oxford. In Shakespeare's time it was known for a certain kind of cheese, which they sliced very thin [one of his skinny characters is called a "Banbury Cheese"] though that's totally forgotten today, and now they're known for a special cake they do, called, not surprisingly, a Banbury cake.)
As for "with knobs on" (or even "with brass knobs on"), that is idiomatic English, though a bit old-fashioned. It puts me at least in mind of the great comic writer P.G. Wodehouse. (A website of his fans advertises a get-together including "sermons, quizzes and games that are truly oojah-cum-spiff with knobs on".) It's mostly used though, including by Wodehouse [pronounced, by the way, "Woodhouse"] in the sense also cited, as in "same to you, with knobs on." It's just a bit of classic oneupsmanship.
You should be careful about using it though, as knob (or rather nob) is nowadays British slang (known somewhat in America) for "penis". So that if you say "I'll be there with knobs on" (or, better yet, "with a knob on"!!).....well, let's just say that chuckles will not go unheard.
But as for the original expression at issue, «συμφωνῶ καὶ ἐπαυξάνω», saying "I couldn't agree more" is, as the English say, "spot on"; though of course you can also just say "absolutely!" "I absolutely agree" and "I agree wholeheartedly" are good, or "you're absolutely right" &c.
More colloquial possibilities (among many) include "amen!" and "I hear you" (which in Black English is "I feel ya"). A bit hippyish but quite common is "right on". To go back to Wodehouse, if you want to sound like Bertie Wooster, you can just answer "quite" or (better, and more emphatic) "rather" (pronounced more or less spondaically). Though you have to have the right tone of voice to pull it off...and to let people know that you're saying it as a joke.
As for Epetelos' "I second this motion", this comes from governmental protocol: if one office-holder has a motion (=proposal) he'd like to put forward, the rules may require that it be seconded, meaning that at least one other member agree to proceed. You might say "I propose that [bla-bla-bla]" and the Speaker will ask "does anyone second the motion?"
A famous song contains a play on this expression: http://www.oldielyrics.com/lyrics/the_temptations_with_the_supremes/i_second_that_emotion.html
(And, speaking of hippies, this song was covered by that legendary group the Εὐγνώμονες Νεκροί.)
....So, there you go, far more information than you were looking for!
P.S. My adopted grandmother in Λυγουριό (in the Argolid) says «ἔλα ντέ» to express emphatic agreement. A friend of mine from Thessalonica, when I used it similarly, said that I was misusing it, but a Μοραΐτισσα friend of mine said that that was correct, but limited to the Peloponnese.