welcome
,
usually all 1/8"(3.5mm) terminated electret condenser mics can be used with the irivers combined mic/line-in which supplies 3-4V to your mics.
I don´t have the coresound cardiods but I had no issues with any of my 6 electret condenser mics in the past when using my iriver.
of course you can also use a battery box between your mics and the iriver´s mic/line-in so your mics get properly powered with 9V (or 12V).
hope you rockboxed your iriver..makes recording very easy.