Thank you for your advice! I have done it, but I think not correct...
import org.apache.commons.io.*;
...
String fileName = "firstGuidedRule.brl";
File file = new File(fileName);
String fileAsString = FileUtils.readFileToString(file);
BRXMLPersistence read = (BRXMLPersistence) BRXMLPersistence.getInstance();
BRDRLPersistence write = (BRDRLPersistence) BRDRLPersistence.getInstance();
String outputDRL = write.marshal(read.unmarshal(fileAsString));
How can I pass it now to PackageBuilder?
(I am sorry for such simple questions..)