There are very strict Federal Laws regarding wild waterfowl. You need to be very careful you do not place yourself in legal jeopardy. Please see the following link for more info. http://www.fws.gov/forms/3-200-9.pdf
Yeah just ONE MORE thing the friggin government needs to be involved in, NOT!!!! ok ok I'm not gonna get started on that (I could go on forever)
You MUST be prepared to COMMIT to the duck! You can Not divorce the duck, you can't decide "oh this is just to much trouble" when it's not so little and cute anymore. IF you commit for real, I just don't see a problem. I'm sure that is gonna get me in trouble with somebody but it's obvious you love the duck and you will probably keep it anyway. As the others said DO NOT turn him out one day after teaching him to depend on you!!!!