Plain simple!
Have own options in Workshops which enables you to craft foreign Weapons and armors (and other usefull stuff such as Shortbows, LongBows etc ) that are not normally used by your civilization due to either size discrepancies or due to knowledge.
The main Idea behind this is that whenever Traders arrive they could ask for some custom made armors and weapons (or other items such as statues, rings etc )that would fit their size (or in case of a statue picture something they ordered ) and would thus enable your fort to gain the basic knowledge to make such weapons and armors (or other stuff) at either normal difficulty or perhaps a slight negative modifier for being foreign (reducing the amount of masterwork items). Over time the knowledge to make such items would pass on to new generations of Smiths / Craftsmen and could also be stored to the civ's knowledge inn books (kinda like how taming now works).
This should also make it possible for dwarves to make some custom fits for armors that their civilization normally would not have access to (designs and such) but they would still be limited when it comes to weapon size and useability.
A slight example :
A human Trader asks for some Well Crafted Iron Chainmail and Leggings and also wants a Masterfully Statue picturing the Decapitation of the Elf I'wash'No , after some troublesome work at the elven statue (basically the workshop was overcrowded with happy dwarves to all times) the statue was finished in time for the next caravan and the human paid a small bonus for the items. The dwarves would now also be able to make these types of items in their workshops under the Foreign menu (Which would depict the item and source of it there) and later they could have access to it directly in their Normal menu wheras applicable (for items such as armors and other wearables that would fit on a dwarf if made smaller).
It would thus enable them to make that type of item for either Humanoids or for Dwarves (or other combos depending on what you play as) and later on pass that information to both your fort and the rest of your civ.