I did that once. Didn't put it on wrong, just customer changed and wanted to add another border after backing was bought. Put it on the long arm and voila, too short. I believe the only way is to take it off the frame, sew the xtra piece on and put back on the machine like you did.
well, I thought a while and went on and took it all off, added to the lining. Put it back on the long arm, lined up the pattern and finished it.I really learn from my mistakes. Take time to measure and it saves time in the long run.