Design pattern for transformer involving third party library

Suppose there is a third party library containing base class Transformer and concrete implementations TransformerA and TransformerB.

I need to write parallel classes for TransformerA and TransformerB outputting class say TransformerNew

public class TransformerAConverter {
  public TransformerNew convert(TransformerA transformerA) {
    // conversion logic
  }
}

public class TransformerBConverter {
  public TransformerNew convert(TransformerB transformerB) {
    // conversion logic
  }
}

I need to write following function:

public TransformerNew[] process(Transformer[] transformers) {
}

How can I achieve this without instanceof or explicit type casting. I have tried using visitor pattern but unable to express it.