[TYPO3] CSV User Import (rs_userimp) not seeing feuser PID

[Tony Lush]

When using the generic CSV User Import tool (rs_userimp) it is not seeing the system folder into which feusers will be stored. 

In the site's main template setup I have:

plugin.tx_srfeuserregister_pi1 { 
..
pid = 51 (id of system folder)
.. }

If I attempt to import into tt_addresses both available storage folders are shown, but when FE (frontend users) is selected, no system folders are available to select from.

Thank you -- Tony