How do I convert a single curtain rod into a wider one?

You can purchase rod extenders which fit between the two parts of the rod. There is the chance the rod will sag due to increased weight, but you can add support as needed.

what is the rod made of?wood,aluminum,wrought iron? By wider do you mean so it stretches farther across the window or so the rod itself would be wider/thicker? using a Tension style shower rod would possibly help with both.certain curtain rods should only be extended so far because they will bow/bend in the middle from extra weight and being stretched too far out.Need a few more details about what you are trying to get accomplished.If you are trying to save on buying new longer rod just get yourself solid longer stick from a tree outside & use as rod,or piece of rope or wire,or long springs would work too.

You can buy the support piece in the rod area.

I got a curtain rod extender at Lowes....fits in between the smaller rod and larger rod...probably extends the whole rod about 8 to 10 inches. You may need a support if your drapes/curtains are heavy.