Pass the krypair of backend wallet as a Signer for the transaction

[umaid56]

I want to sign the transaction using both a keypair as a backend wallet and the Phantom wallet. I am using the Solana wallet adapter package. How can I do this with the Solana wallet adapter and the backend keypair? I was using this but it can't work as it give me error

Error:
transaction.serialize is not a function at StandardWalletAdapter.sendTransaction (adapter.ts:347:1)

const backend = Keypair.fromSecretKey( base58.decode(keypair ) );

const ExecuteInstructions = await constructExecuteSaleTxByBuyer(wallet?.adapter,publicKey,sellerPubKey,mintAddress,1)
     ExecuteInstructions.recentBlockhash = (
   await connection.getLatestBlockhash()
    ).blockhash;
   ExecuteInstructions.feePayer = publicKey

   ExecuteInstructions.sign(backend);
   const serializedTx = ExecuteInstructions.serialize({ requireAllSignatures: false });
   const ExecuteSignature = await sendTransaction(serializedTx, connection);
   await connection.confirmTransaction(ExecuteSignature);
   console.log("\n\n\n\nExecution Done",ExecuteSignature)
   ```