If your package already has an MSI, it does not need to be repackaged. The repackaging process is for legacy installations (.exe) which are not in Windows Installer format, but need to be.
Since your installation is already in Windows Installer/MSI format, you can customize without modifying your original MSI by creating a transform. In AdminStudio, you can use the Tuner tool to create a transform. For example, in Tuner's Setup Properties view, you can specify the ProductLanguage property so that your installation is done in English.
Once you have your transform, you can run the installation silently, with the transform, with the following commandline:
msiexec /i
/T /qn
If you are having specific issues either with AdminStudio Tuner or with certain validation errors, please send an e-mail with more complete details to nank@installshield.com