It's fairly simple coding. You can "fuse" the internal SSD with an USB SSD with you want it doesn't need to be internal.
In terminal do the following:
Note down the disks you will use to the Fusion. Remember that those disks will be erased, so keep a copy of your stuff. As for example, you have to note the "Disk4" that will appear in the right of the list. You will note down the internal SSD and the external one. So you will have two Disks.
Diskutil erasedisk JHFS+ Bla /dev/disk4
Here we will erase the disks. The point here is to eliminate any garbage partitions from the disks, and set their GPT partitions. Do this for the two ones. "Bla" is the temporary name of the disks. You can enter whatever you want. It doesn't matter.
Diskutil apfs createContainer -main disk0s2 -secondary disk4s2
Here we will Fuse down the drives. Remember to replace the disks number with the ones that you have noted. The main disk will be your Internal SSD, the Secondary, the USB SSD. Beware to include "s2" after the disk number, as this operation needs to be done with partitions, not entire disks. If you have run the second step, your main partitions will be s2.
After the operation is complete, it will give you a new Disk number for the fusion drive. Take a note of it.
Diskutil add volume disk6 APFS Name
Here you will put the Disk created. Remember to Name it as you want. This will be the name of your fusion drive.
Enjoy your brand new Fusion Drive. This drive will be able to boot Mojave and High Sierra with no worries.