The shape of those 95W-max-TDP Z600 "Mainstream" heatsinks/fans is offset so the memory cooling shroud just barely fits. I doubt a generic heatsink/fan will fit properly, and you don't want to throw away that cooling benefit for your memory.
As an alternative you could look for a 130W-max-TDP "Performance" fan from the Z400 which will just fit over one (but not both) of the Z600 sockets if you slightly bend down the silver rear air deflector. I have done this to check, and it does.
The Z600 workstation will not complain about having 1 Mainstream and 1 Performance heatsink/fan to cool both 95W processors. The Performance heatsink/fan made specifically for the Z600 are rare and quite costly. Performance wise I am very happy with my current 95W X5765 processors. There are many more of the Z400 Performance fans out there, rated for the hotter running 130W processors that most of the Z400s use. Those go for about $30.00 USD used off eBay here, including shipping.
Be careful buying a non-HP heatsink/fan also because HP has tuned its fans to the PWM control system of its motherboards.