Merging B.net accounts only occurred when they made the switch over to Battle.net. Currently there's no way to merge two separate B.net accounts.
You can, however, add a WoW license into your B.net account, but only if you are related to the other account holder IIRC. A married couple can link accounts, as can a parent and child. I think you'd need to prove that you are related somehow, though.
I've never tried to add someone else's WoW license to my B.net before, so perhaps someone with more experience in this area will stop by and give you more details.
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