I sent a feature request to Synology about being able to create volumnes on NVMe-devices (found currently as cache on my DS918+), but they stated that they have no plans to do so:
"We do not consider support this feature, because NVMe SSD is very hot, especially in sequential use, so it is easy to shut down due to overheating."
Also from Synology Support, clarification regarding future NVMe support on the DS3617xs:
Please note that the limitation isn't necessarily hardware-related. The limitation is due to the Linux kernel version, which only a handful of units are using a version with NVMe support. Unfortunately there are no known plans at this time for NVMe support on the units currently lacking it, or any information on when the kernel may be updated.
To clarify in light of NAS Compares recent posting about a DS3617xs II, will the current DS3617xs support NVMe in DSM7? Will existing DS3617xs then be upgradeable to a "II" level. Or will purchase of a new "II" level NAS be required? Maybe too soon to know; but thanks for any further insights...