OOPS! Should have tried on the choker I was making before I added the bar/ring toggle, crimped, and cut the wire--it is too tight! Any suggestions for "extending" the length of a necklace without removing the toggle? I'd hate to re-string it as it was a very intricate (time-consuming) project. Next time, I'll measure twice, cut once!
Add another clasp to the clasp already in place and consider using a multiple ring toggle clasp to allow you even more sizing options. Connect the toggle bar half of the new clasp and fit it into the ring of the toggle clasp already in place. Connect the toggle bar from the existing clasp and fit it into the appropriate ring of the rings from the next clasp.
If the new toggle clasp doesn't quite work together based on size (you always need your toggle bar to be longer than the width of the ring so it doesn't fall through easily) use jumprings to connect the new clasp to the existing clasp.
If you find yourself in a similar situation for a design that doesn't have a toggle clasp, consider adding a extender chain. A complementary sized lobster claw clasp is perfectly geared to this type of extension.