Synology won’t use a USB-SSD as source for backup, so you need a mount
Create a shared folder on Synology NAS,
Via File Station on Synology DSM
Create Shared Folder, e.g usb_source
sudo mount --bind /volumeUSB1/usbshare /volume1/usb_sourceCopy
Now you can use the usb_source shared folder (which is mounted to the USB-SSD) as backup source