A possible (temporary) solution would be to use Virtual Machine Manager in Synology, and install DSM6.x within that, and then install Nakivo there, setting up a repo within DSM7, which nakivo (within DSM 6) can then mount. When Synology gets around to approve/whatever nakivo, then you can move nakivo to DSM7, utilizing that same repo.
It would be nice to know if Synology approving Nakivo is going to take days, weeks, months... But there is just no way to know. So who knows if we need to do this workaround or just wait "one more day".
Synology does give you ONE license to use a virtual DSM within VMM.
Go to https://www.synology.com/en-us/support/download/VirtualDSM#system and download the latest 6.x VirtualDSM
Within Virtual Machine Manager, go to Image (on left), then DSM Image (top), then Add, and upload the virtual DSM you just downloaded.
Next, go to Virtual Machine, Create, and for Operating System select "Synology Virtual DSM", etc.. hopefully you can figure out the rest.
Once the virtual DSM is up and running, you can log into it, and go right to package center and download Nakivo.