The Necromancer pack for Diablo 3 will be released in 2017, and you need to spend real money to buy it. It is however possible to play WoW to earn battle.net account balance, which can be used to buy anything from Battle.net, including the Necromancer pack for Diablo 3.
What you need to do to earn battle.net credits on WoW is to play and earn gold. Once you have a lot, you can buy an item called WoW Token, which can be used to extend your WoW game time, or to add $15 to your battle.net account balance. The thing is that these items are not really all that expensive on the auction house, and the price usually varies between 50k and 100k per token. Getting this amount will not take a lot of time if you follow some good gold making guides.