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Use the private key they provided, and after you have created your Peerplays account issue the "import_balance account name ["privately"] true true" command

is there a way I can just use my already existing bitshares wallet

Yes, but you needed to own bitshares on your wallet prior to the snapshot that took place on Sept 2016. Use import_keys "bts-youracountname" privatekey to import your bitshares account.