Yes, you can use any kind of knife you want to use even a plastic one as your athame. You are the only tool that's essential, after that any tool you want to use is a-ok as long as YOU have no issues with using it. Some people are rule bound and will never be happy with an athame that is not a double bladed black handled knife, but if that's not you, then there's no problem.
As to your second question can a christian be a wiccan. The answer is it depends on what kind of christian you are. If you are the type of christian who believes that only praying on your knees is kosher, that the KJV must be read literally, and that Jesus objects to everything but singing The Old Rugged Cross then you probably can't be christian and wiccan.
There is, however, a relatively "new" syncretic wiccan tradition called Trinitarian Wicca and those wiccans do claim to successfully combine wicca with christianity. My understanding is that they combine wiccan ritual with a belief in the christian trinity. Since wicca is a religion with no sacred dogma, wiccans across the wicca spectrum share similar ritual practices, and a similar world view, but they are not obligated to share the same gods. So although there is a certain segment of the wiccan population who will tell you it "cannot" be done that is their personal opinion since there are wiccans who are doing it successfully every day.
If this is your cup of tea don't listen to the nay-sayers. You are entitled to call yourself Trinitarian Wiccan or Christian Wiccan if that's what you do and you are also entitled to have your choices treated with respect especially if no one can point to a written wiccan rule that your choices clearly violate.
All these many years after R.Buckland brought self-dedication to the USA there are still Trad wiccans who insist that non-trad wiccans or eclectic wiccans or solitary wiccans are not REAL wiccans when indeed they not only are REAL, they are accepted as "REAL" by all but a bigoted minority. In other words some people just feel the need to claim that they are the ONLY REAL DEAL. Christianity has those kinds of people. Islam has those kinds of people, Hinduism has those kinds of people. It shouldn't really be surprising that Wicca has those kinds of people too. Stick to your guns. In 20 years time, Trinitarians will be just another interesting patch in the wiccan quilt; some newer tradition will be in the Hot Seat, and "everybody" will be poo-pooing them instead of you.