I wouldn't mind lending a hand.
Using any two metal to make the alien plastic or alloy will result in a lot of reactions, especially if you have reactions for both the ores and bars. Each will need its own name, too. You might want to limit it to major metals or some other process.
Still, making the reactions is simple enough. Check reaction_smelter.txt in the raws for how to convert from one metal to the other. Examples using the brass reactions as a template:
Two different metal ores:
[REACTION:MAKE_ALIEN_ALLOY]
[NAME:make alien alloy (use iron + copper ore)]
[BUILDING:SMELTER:NONE]
[REAGENT:A:1:METAL_ORE:IRON]
[REAGENT:B:1:METAL_ORE:COPPER]
[PRODUCT:100:2:BAR:NO_SUBTYPE:METAL:ALIEN_ALLOY][PRODUCT_DIMENSION:150]
[FUEL]
[SKILL:SMELT]
Two different metal bars:
[REACTION:MAKE_ALIEN_ALLOY2]
[NAME:make alien alloy (use iron + copper bars)]
[BUILDING:SMELTER:NONE]
[REAGENT:A:150:BAR:NO_SUBTYPE:METAL:IRON]
[REAGENT:B:150:BAR:NO_SUBTYPE:METAL:COPPER]
[PRODUCT:100:2:BAR:NO_SUBTYPE:METAL:ALIEN_ALLOY][PRODUCT_DIMENSION:150]
[FUEL]
[SKILL:SMELT]
Two identical metal bars:
[REACTION:MAKE_ALIEN_ALLOY3]
[NAME:make alien alloy (use iron bars)]
[BUILDING:SMELTER:NONE]
[REAGENT:A:300:BAR:NO_SUBTYPE:METAL:IRON]
[PRODUCT:100:2:BAR:NO_SUBTYPE:METAL:ALIEN_ALLOY][PRODUCT_DIMENSION:150]
[FUEL]
[SKILL:SMELT]
With these done you just need to make a new metal for the alloy, like so (template is the iron entry in inorganic_metal, unchanged except for the names):
[INORGANIC:ALIEN_ALLOY]
[USE_MATERIAL_TEMPLATE:METAL_TEMPLATE]
[STATE_NAME_ADJ:ALL_SOLID:alien alloy]
[STATE_NAME_ADJ:LIQUID:molten alien alloy]
[STATE_NAME_ADJ:GAS:boiling alien alloy]
[DISPLAY_COLOR:0:7:1]
[MATERIAL_VALUE:10]
[SPEC_HEAT:450]
[MELTING_POINT:12768]
[BOILING_POINT:15150]
[ITEMS_WEAPON][ITEMS_WEAPON_RANGED][ITEMS_AMMO][ITEMS_DIGGER][ITEMS_ARMOR][ITEMS_ANVIL]
[ITEMS_HARD]
[ITEMS_METAL]
[ITEMS_BARRED]
[ITEMS_SCALED]
[SOLID_DENSITY:7850]
[LIQUID_DENSITY:6980]
[MOLAR_MASS:55845]
[IMPACT_YIELD:542500] Was 1080000, but just using 3.5x tensile multiples for everything until better numbers are available, which might not be likely
[IMPACT_FRACTURE:1085000]
[IMPACT_STRAIN_AT_YIELD:319]
[COMPRESSIVE_YIELD:542500]
[COMPRESSIVE_FRACTURE:1085000]
[COMPRESSIVE_STRAIN_AT_YIELD:319] bulk modulus 170 GPa
[TENSILE_YIELD:155000]
[TENSILE_FRACTURE:310000]
[TENSILE_STRAIN_AT_YIELD:73] young's modulus 211 GPa
[TORSION_YIELD:155000]
[TORSION_FRACTURE:310000]
[TORSION_STRAIN_AT_YIELD:189]
[SHEAR_YIELD:155000]
[SHEAR_FRACTURE:310000]
[SHEAR_STRAIN_AT_YIELD:189] shear modulus 82 GPa
[BENDING_YIELD:155000]
[BENDING_FRACTURE:310000]
[BENDING_STRAIN_AT_YIELD:73]
[MAX_EDGE:10000]
With these in place, you need to add all of the new reactions to the alien entity. Look in entity_default and search for PERMITTED_REACTION to see where they go within an entity.
[PERMITTED_REACTION:MAKE_ALIEN_ALLOY]
[PERMITTED_REACTION:MAKE_ALIEN_ALLOY2]
[PERMITTED_REACTION:MAKE_ALIEN_ALLOY3]
I recommend you create new files to house your mod data, rather than simply adding it to the existing raws. Keeps things tidier and much easier to locate!
With these in place, you should see three new reactions in the smelter. I didn't test it, though, so I may have forgotten something. Still learning the modding ropes myself!