Neat little catch 22 I've discovered in setting up Portal 6 on our iSeries. Both of these statements are from the InfoCenter. It appears that there is no clear way to install the Process server components in a clustered environment (or a managed) on an iSeries from an initial or scripted install.
i5/OS users: Installation of WebSphere Portal is not supported on a federated node in an i5/OS environment. If you are building a cluster on i5/OS, use the instructions in the following section to install the portal and then federate the node: Installing WebSphere Portal on an unmanaged node (primary).
Remember: Because the default installation of WebSphere Portal, including business process support, is not supported for installation to an unmanaged node and later federation, you must install WebSphere Portal without business process support, as described in the installation instructions.
You can however, use the BP support from a remote server, you would just need to install the Process server separately into its own federated profile (and preferrably on its own gear). Then you run the Portal bpe-unconfig task to unconfigure it, then install the Member Manager plugin. Full instructions are in the following link: http://publib.boulder.ibm.com/infocenter/wpdoc/v6r0/topic/com.ibm.wp.ent.doc/wps/bpi_admin.html