How do I merge an external SSD with my internal SSD?

I have a late 2015 5k iMac, that unfortunately has a measly 256GB internal SSD. I mainly work with photos, videos and extensive media (music and movies) library.

I recently purchased a 1.92tb Sansdisk Extreme 900 SSD which is blazingly fast and sits behind my iMac. Is there a way, or even a benefit, to merge these two drives so they show up as one larger drive on my iMac? The original internal SSD is filling up at great speed, I'm currently storing my media on the external, but would love it if they could work together rather than flip-flopping between the two.

I understand that perhaps Fusion drive is an option, which I have done before with an HDD and SSD, but I don't know if that is helpful in this situation.

Any advice would be much appreciated, I want to know which option is best before I start erasing and restoring any drives!

Thank you,