To change cartridges before their "empty" - use the windows software (windows is still useful for something!!)
Ok! It seems that it is indeed possible to access the second DDR slot. The only thing one has to do is to remove the keyboard, and that's it!! It's beneath it:
http://hardware.mcse.ms/message92524.html
check also the detailed Maintenance and Service Guide for zt3000, nx7000, etc