Yes it is.
It is not possible to install standard PC and server memory, I've never seen a system of any brand which supports such a configuration.
Officially, the Z400 supports only ECC unbuffered memory modules.
However, it will also run with standard DDR3 RAMs, unsupported only means that HP Customer Service won't help you.
There are some reports that even 8 GB Modules work, but that's another story, 24 GB runs fine.
Mixing different types of memory is generally a bad idea.
Use either ECC unbuffered (which is recommended) or non-ECC, preferably six identical modules.
DDR3 are relatively cheap, even those with error checking and correction.
Good luck
