How to import a Raiblocks era private key/seed?

I have a private key like this: 01234567...ABCDEF123 that I purchased many years ago. I now downloaded the wallet Natrium and it uses a 24 words mnemonic key just like bitcoin. How to I import or more to this new private key?

With Natrium there should be a little icon in the top-right corner that looks like a sprouting seed or similar, if you click that you should be able to use the 64 character hex seed.