well the idea is to install a normal Debian on your system and transform it into a DietPi later on.
Another way would be to remove the SSD from your computer and connect it using USB to another computer and flash DietPi directly on the SSD.
But yes this all would mean to run a new installation
If you like to transfer you current installation (boot partition), you could have a look to this post
https://dietpi.com/forum/t/move-boot-partition-to-smaller-sd-card/3687/7
But it was not intended for your case. So some parts would need to adjusted to fit your need. And no guarantee that it will work. Much more clean would be a new install directly on your SSD.
Keep in mind that you currently have 2 partitions. BootFS + RootFS. The RootFS can be transferred later on to a separate partition using Drive Manager.