Ok, I found a fix and a potential workaround for this:
I successfully used the fix proposed in this French topic:
https://xpenology.com/forum/topic/18601-tuto-hp-n54l-gen7-dsm-7-et-dsm-623/
Which is translated to English here:
https://xpenology-com.translate.goog/forum/topic/18601-tuto-hp-n54l-gen7-dsm-7-et-dsm-623/page/1/?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=nl#comments
So you need this driver: ohci-hcd.ko and DS3615XS. I used DSM 7.1 and it works great for me.
With these instructions you can complete the process: https://mariushosting.com/synology-how-to-add-usb-support-on-dsm-7/
The workaround I have not completely tested, but it could work:
The cause of this issue is that DSM7 does not have a driver for the HP N54L Microserver USB 1.1 ports. So you see the 480Mbit devices, but not the 12Mbit. If you connect a USB2 hub to the server and connect the 12Mbit devices like Zwave, Zigbee sticks, they should show up with lsusb.