If you own the copyright to that code, it's not clear to me why putting it in the code you're giving to them would necessarily even give them the right to use it, much less give them the copyright to it.
Meaning: if I were your prospective employer, I'd be more
(a) concerned that your work product contains code that I do not have clear title to than
(b) hopeful that I will somehow grab the copyright of your pre-existing work.
You'll be a lot better off if the arrangement is clear beforehand, and don't sign anything that would alter or undermine that arrangement. People will have more respect for your being above-board and straightforward, and for the fact that you have enough self-respect to assert your proper rights while making a fair bargain.
Meaning: if I were your prospective employer, I'd be more
(a) concerned that your work product contains code that I do not have clear title to than
(b) hopeful that I will somehow grab the copyright of your pre-existing work.
You'll be a lot better off if the arrangement is clear beforehand, and don't sign anything that would alter or undermine that arrangement. People will have more respect for your being above-board and straightforward, and for the fact that you have enough self-respect to assert your proper rights while making a fair bargain.