Go to your original posting of the barn-raising. Right-click the link that someone would click to help, and select "copy link". Then make a new post asking for help, and paste this link into it, inviting your neighbors to help again. It looks ugly and doesn't have the pretty picture, but it works. I usually do it as part of a different FV post.
Note that friends can help once a day, so even though you only have 3 days to complete a barn-raising it doesn't have to be ten separate friends that help. Helpers receive the bonus again if they help again.