No, unfortunately, you cannot load gift card funds onto a magic band. You can use the charging feature of the magic band, if you have a resort reservation and can then pay off the charges with the gift card at your resort. But preloading cannot be done.
You can split magic bands onto different credit cards. We've done it in the past. Some CMs will let you "prepay" your account with a gift card. Depends who you get and how knowledgeable they are. Otherwise you can just go at the end of the day and pay off the charges with a gift card.