Extender Plus was actually part of David Ikeda's Power Tools. Newtek integrated the plugs to lw8 so now you cannot download them however there are a few other plugs that work in the same way that are still available.
http://www.davidikeda.com/ditools/index.html
http://www.pictrix.jp/lw/index2.html
Have a look on
www.flay.com to find more options. Super Shift, Smooth Shift and bevel work in much the same way as extender plus when working with polys.