Weapons have mod slot limitations based on their model. If Steam's Mythic swords are already using the maximum number of slots you'd be unable to add more - at the very least without some issues.
Still, generally, you'll first need to tweak the field that defines max mod numbers and types. Then you can tweak the field that defines what mods the item can roll for, which will be informed by it. Eg, you'll first need to specify that an item can get up to 3 relics and up to 3 fuels before being able to set it up to roll, say, 1-3 relics and 2-3 fuels. If you only do the latter it'll be an impossible outcome, since you're pushing beyond its limitations.