There are three types of IM packages:

ADF These are recording style packages. They include the source files and folder structure required to install the application and a file with the wfs extention. This holds the destination and copy information for the files as well as registry modiication information.
ASF files are easily modified to include additional scripts, files and registry modifications.

MSI files are supportes as well as their mst modifcations to the installation. You can use various utilities to create custom installations using resource kits or by directly changing the MSI file through ORCA from the resource kit.

MSP files are also supported these are used frequently to update applications that use the installer service to update installations.