Kairi didn't had any training in using the keyblade until riku gave her one that why I think Namine didn't use a keyblade
and for Xemnas I her that the director ( I think ) said in an interview that Xemnas can use the keyblade but he didn't want to use it for some reason
and for the way to obtain a keyblade I know of two :
1- the Keyblade Inheritance Ceremony that how the three heroes got their keyblade ( Riku from Terra / kairi from Aqua / Sora from Riku when the island was destroyed )
2- like the keyblade worriers before the keyblade war who made their keyblade in the shape of the X-blade
I'm not sure if there is another way