1) Is it possible to pay khums on an item before it has become unused 1 year? Does it make it not khums liable for ever in the future?

2) If my wife is someone who does not earn for herself and I support her financially, is it possible for me to pay the khums on her unused items?

1- Yes, for both assumptions.
2- You can grant her the money as gift, and then she pays it as khoms.