Do you think that with enugh research, humans could develop tails, or maybe scientists could find a way to implant one?

Yes might be possible in the future but why would humans want one? It would imbalance us for walking and running and as land dwellers we have no need to have them as an aide to tree climbing.

We do of course have tails, or rather the vestiges of one. We still have some tail (caudal) vertebrae in our bodies, but not enough to to show externally. Occasionally though some people are born with an abnormally high number of bones and an apparent tail:

http://funstuffcafe.com/img/strange-wei … tail01.jpg