Whenever the human/dorf caravan rolls around, buy out ALL the metal items. If $ is an issue skip hte highly decorated/trap components as they are very pricey. You can also directly request bars, ores, and crafts/weapons to smelt down. However, importing items won't fufill a mandate. You have to melt down and make on site. However, by prioritizing metal items from trade and being frugal with the bars, you can stretch them a loooong ways. When I know I am embarking on low-metal areas I tend to start from the very first caravan. Keep in mind that most items are melted down in fractions (.3 of a bar, .7 of a bar, etc). These fractions are actually collected in the smelter. Whenever it ticks over 1, you get a full bar. For this reason its best to limit the number of "recycling" furnaces so you don't wind up with several fractions of 10 different metals across all your furnaces.
If you are feeling exploity... several metal items melt down for an INCREASE in bars. Picks, for example, require one bar to make but are melted down into 1.2 bars. Thus, for every 5 picks you melt of hte same metal you get a free bar. Some items like "menacing spikes" and "giant axe blades" melt down for 1.5 bars when they only take one.... its redonkulous. So if you really wanna go dark side with metals, you can get a single unit of (imported) steel and go crazy making menacing spikes to melt down. Using magma you give up no resources except dorf labor. However this trains a skill (and attributes).