Why not use a contrasting solid? I think using all stripe wouldn't work so well.

And then you need to consider whether an 18 inch addition to an almost 12 foot long drape would look skimpy - unbalanced in relation to the larger section.

You could add the same stripe to the ready made panel to make the panel long enough. Depending on the size of the stripes, matching could be an issue. I agree with graywings that adding only 18" might be create an off balance look. For a block panel, I think the color/fabric change should line up with the window sill.The seam will be noticeable-that's why you see a decorative ribbon or gimp covering the seam. If you can tackle adding a seam to an existing drape, you could probably sew the entire panel. Cahnging a ready made is time consuming.

I don't think adding a contrasting solid at the bottom would work when the rest of the curtain is striped. Now, if we were talking about adding a contrasting solid to curtains that were also solid, then that might work.

If you add the same block of striped block of fabric and lined the stripes up, I'm curious to see how it would look. Would somebody be able to notice that you needed to do that to the curtains?

I have a similar problem, and my best option might be to do something similar. Although, I'd love to see examples of that first to see how noticeable it would look.

One thing I do think might be a problem is that you'd have to cover up the seams with ribbon or gimp, and so you'd have this horizontal line running perpendicular to vertical strikes. That might look a bit too busy, with vertical and horizontal stripes clashing.