I have a twin size headboard and footboard, but an extra long boxsprings for a twin bed. How do I extend the metal bed rails so my boxsprings will fit in the frame?

It sounds like what you actually have is half of a king size box springs.

On a king bed, the box springs are two separate units that sit side by side to support the mattress.

I don't think you will be able to use your footboard, with the king box spring, without trying to modify the frame on your own.

I know of no kit or extension to accomplish this task.

You might find something useful to you by clicking on the following search.

Bed Rail Extenders - Google Search