System X

No. You are unable to utilise ECC and ECC registered RAM in your IBM workstation at the same time. If you are installing new RAM alongside the existing memory you need to be sure to match the specs exactly. ECC non-registered type of RAM is always constructed with a x8 chip configuration and ECC registered RAM may be assembled with both a x4 and x8 configuration. 

However in most cases you still cannot even mix ECC registered RAM if the chip types don't match. If your system has ECC registered x4 chip, then you must match those specifications exactly. If the system currently has ECC registered x8 chip type of RAM, then that is what you must install in order for the system to work.

If the existing RAM is just ECC type and rather small capacities(1g or 2gb modules), then it is usually better value to simply bin the existing RAM and utilise ECC registered x4 type. They are the best value per GB and installing 4x8gb or 4x16gb or this type is far less costly then trying to install a bunch of small capacity ECC only type.