Looking for cheap ways to extend steel necklaces

I've got a lot of second hand necklaces from a friend who doesn't enjoy them anymore, but we have a similar style. However they're all a bit shorter than I like. What's a frugal way to extend them?

I've found some necklace extenders online, but they seem to be targeted more at jewelry makers so you have to order in bulk. On top of that they necklaces are all stainless or surgical steel, and most of the extensions online are silver or gold, which seems like a waste lol