How to add material to curtains to make them longer looking

Try either a toning or contrasting border top or bottom if you want to add length

You can add a layer of lace at the bottom of the curtains. Also, you can cut the existing curtains in half and add another contrasting material there. Another options are adding a border with different color and adding tassels.

Put material on the top of the curtains, it's less noticeable. I don't know your color scheme, so you have to decide if a pattern or a color would look best. Try to make the add-on, a color or a print, so you have some contrast.

Cut the curtains about 1 foot from hem add extra fabric then replace bottom hemmed curtain back. Tidy exposed sides .............. I you want it to look even more co-ordinated, you could repeat process using smaller inserts higher up. You could also use buttons to trim at top or down sides.........

Perhaps you could cut them in two several inches up from the bottom and add (sew in) the required amount (inches) of material in a contrasting color.

A simple way is to add tabs to the top to lengthen them or add trim to the bottom, like fringe.