It's not the only way, you could do it by tracking down the entry in the rdb and copying the header to the cht file, or manually adding the crc and such, or whatever. But the easiest/most reliable (ie. no chance of putting cheats in for non-existant/false crc) is to have the actual rom. Adding it without the rom is kinda silly anyway, you'd have no way to test if the codes are valid or not.