I accidentally made a BCH withdrawal to my BTC adress instead of sending it to my BCH adress. So i won't surprise you i didn't receive it yet.
Well several newbies recently have been making this mistake and sending BCH to a BTC wallet. Your coins will be lost forever if you do so. Since both of them are different currencies and they are not same you cannot "mix" the two.
Is there anything i can do about this? I read something about using a privatekey on your BTC adress to claim your BCH? Is this possible and how can i realise this?
I made the withdrawal from wex.nz to my bitpay account. The support team of wex sent me an email with the link of the transaction on blockchair. I also send an email to bitpay for support however i didn't receive a respons from them yet.
You should contact support on wex.nz and bitpay explaining your situations to both. It depends on the website to take care of the issue. You cannot do anything about it. Let this be the first and the last lesson for you. Never send BCH to BTC wallet or vice versa, Never send OMNI tokens to Ethereum wallet and so on.
Chances are that it can be claimed back. Do as TheUltraElite advised you. If you were in a control of the private keys (a wallet on your computer, a full node ideally) your chances would be higher, but still I would try to contact them.
Whatever happens, and especially with TheUltraElite's comment on "several newbies" in the wider context, maybe it will console you to some extent to hear that the BCH team is (as I write this) working on the hard fork that will change their address format to make the mistake like yours impossible in the future...