Hi Ladyath,
Are you talking about merging multiple XML forms into one XML form using the File->Save&Send->Merge Forms option? If so, you'll need to make sure each original form has the username in it. Then, change the XSLT file associated with the ImportParameters node in your manifest.xsf. You can use your favorite XML editor or Visual Studio to edit the XSLT. The XSLT is a transform file that describes which fields will be "imported" (i.e. merged) from the source files into the destination file.
Best of luck,