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If you have an existing secret key and you want to derive another key from it using PBKDF2 with HMAC SHA-256, you can follow this approach. Keep in mind that PBKDF2 is typically used for password-based key derivation, so the key you start with should be strong enough.

Here's an example in Java:

```java
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.PBEKeySpec;
import java.security.NoSuchAlgorithmException;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.KeySpec;
import java.util.Arrays;

public class PBKDF2KeyDerivationExample {

    public static void main(String[] args) throws NoSuchAlgorithmException, InvalidKeySpecException {
        // Replace with your existing secret key
        byte[] existingSecretKey = hexStringToByteArray("yourExistingSecretKeyInHex");

        // Salt for PBKDF2
        byte[] salt = generateSalt();

        // PBKDF2 parameters
        int iterationCount = 10000; // You can adjust this based on your security requirements
        int keyLength = 32; // for a 256-bit key

        // Use PBKDF2 to derive a new key
        SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256");
        KeySpec spec = new PBEKeySpec(byteArrayToCharArray(existingSecretKey), salt, iterationCount, keyLength * 8);
        byte[] derivedKey = factory.generateSecret(spec).getEncoded();

        // Print the derived key
        System.out.println("Derived Key: " + byteArrayToHexString(derivedKey));
    }

    private static byte[] generateSalt() {
        // Replace this with a secure method of generating salt
        // For example, you can use SecureRandom:
        // SecureRandom random = new SecureRandom();
        // byte[] salt = new byte[16];
        // random.nextBytes(salt);
        // return salt;
        return hexStringToByteArray("yourSaltInHex");
    }

    private static byte[] hexStringToByteArray(String hex) {
        int len = hex.length();
        byte[] data = new byte[len / 2];
        for (int i = 0; i < len; i += 2) {
            data[i / 2] = (byte) ((Character.digit(hex.charAt(i), 16) << 4)
                    + Character.digit(hex.charAt(i + 1), 16));
        }
        return data;
    }

    private static char[] byteArrayToCharArray(byte[] byteArray) {
        char[] charArray = new char[byteArray.length];
        for (int i = 0; i < byteArray.length; i++) {
            charArray[i] = (char) (byteArray[i] & 0xFF);
        }
        return charArray;
    }

    private static String byteArrayToHexString(byte[] byteArray) {
        StringBuilder result = new StringBuilder();
        for (byte b : byteArray) {
            result.append(String.format("%02X", b));
        }
        return result.toString();
    }
}
```

Replace "yourExistingSecretKeyInHex" and "yourSaltInHex" with your actual existing secret key and a random salt, respectively. This example will derive a new key using PBKDF2 with HMAC SHA-256.