JkDefrag does not need a plugin to be run from BartPE, you can simply boot from a standard BartPE disk and run JkDefrag. Maybe you are referring to the special Gui by Emiel Wieldraaijer for BartPE, see
http://www.emro.nl/freeware/, the "JkDefrag GUI version 1.00 for BartPE/XPE Multilangual".