Preferably in English and not for the 817ND!
Every link I've ever been given is response to this question elsewhere actually refers to the 817ND and no, they aren't the same.
I finally found a FT-818ND specific resource.
This page includes a MARS/CAP TX-RX modification for the FT-818ND :
https://www.mods.dk/index.php?ModelId=1274&ModelRec=ft-818&RadioId=1-yaesu
According to the article, the modification is done through SMD jumpers/diodes on the main board, and the exact procedure may depend on the regional version of the radio.
Be careful though: some FT-817 information partially applies to the 818, but not all board layouts are identical. A backup of current settings and a full reset after the mod are usually recommended.