(1) Create the metal, by for example copying the entry for INORGANIC:IRON and tweaking the values to taste. The only thing you really *must* change is the name. If you don't know what the tokens mean the best is to wiki it
http://dwarffortresswiki.org/index.php/Material_definition_tokenYour new entry must be in a file whose first token is [OBJECT:INORGANIC]. This could be the original file that Iron is defined in within your DF install, but for organizational purpose you might prefer to make a new one.
Decide what workshop will do this reaction, if any. If you only want non-playable goblin civs to do this it doesn't need a workshop. Not all vanilla DF buildings are moddable, so you'll have to look into that, or define your own new workshop.
(2) Create the smelting reaction. It will look something like this.
[REACTION:ORICHALCUM_ALLOY]
[NAME:refine orichalcum bars]
[BUILDING:ANCIENT_FOUNDRY:CUSTOM_O]
[BUILDING:MAGMA_ANCIENT_FOUNDRY:CUSTOM_O]
[REAGENT:A:150:BAR:NO_SUBTYPE:METAL:STEEL_DROW]
[REAGENT:B:150:BAR:NO_SUBTYPE:METAL:IRON]
[REAGENT:C:150:BAR:NO_SUBTYPE:METAL:SILVER]
[PRODUCT:100:3:BAR:NO_SUBTYPE:METAL:ORICHALCUM_ORC][PRODUCT_DIMENSION:150]
[FUEL]
[SKILL:SMELT]
I dunno what your comfort level is with modding, so not sure in how much detail you would like this explained. Anything you don't understand look up on the wiki or ask about. Obviously if you don't want a workshop to be required, delete the "Building" lines. The 150's are the required unit of measure for bars.
(3) Now you need to Permit the reaction for whichever entities (civilizations) you want to have access to it. For example, for my reaction above the token is
[PERMITTED_REACTION:ORICHALCUM_ALLOY]. You put that token anywhere under the corresponding [ENTITY: ] entry(s) that you would like to have it. For Goblins that would be [ENTITY:EVIL]. If you look at the existing entities you should see how it works. For bars, that's all you have to do; if they have the smelting reaction they should be able to make and use the metal.