Sometimes it is possible, even in this late period of history, for the Ruach of a righteous tzadik, even from one of the Forefathers, to come as an ibur. It will all depend upon the level of the mitzvot being performed by the person. Some mitzvot have the power to draw down the Nefesh of a righteous tzadik into ibur, whereas others can draw down the Ruach of the tzadik.
It is also possible for a person to receive the Nefesh of one righteous person, and after that merit another Nefesh from another righteous person, even greater than the first. In such a case, he will have his own Nefesh, the Nefesh of the first righteous person as his Ruach, and the second, higher Nefesh, acting in the place of his Neshama.
Thus, in one lifetime he can merit a Ruach and Neshama, though not his own.
Or, perhaps, the Nefesh will be perfected to the point that after already receiving the Nefesh of a righteous person, he will merit the Ruach of a second righteous person, even possibly the Ruach of Abraham the Patriarch! There is no generation in which there is not someone like Abraham, Isaac, Jacob, Moses, Samuel…
In this case, the Ruach of the second tzadik will act as his Neshama, and the Nefesh of the first tzadik will act as his Ruach.
This is the inner meaning of what they wrote in the Midrashim, in particular Midrash Shmuel: There is no generation in which there is not someone like Abraham, Isaac, Jacob, Moses, Samuel, etc. (Bereishit Rabba 56).
In other words, there are people in every generation who may actually have the Nefesh or Ruach of these great people from the past.
There is no pen that will prove sufficient to record all the details in a book. Nevertheless, an understanding person will comprehend and make the necessary inferences on his own.
In summary, based upon the tikun and purification of a particular Nefesh, it can even merit a Neshama from the earliest generations, including the most elevated of all; and it can happen even in this generation of ours.
Furthermore, it is exactly the same when a Nefesh and Ruach reincarnate together and become rectified, but are unable to acquire their Neshama without first dying and reincarnating. They can receive the Nefesh, Ruach, or Neshama of a righteous tzadik as an ibur, and it will act as their Neshama. All the details that were described in the case of the Nefesh alone that has completed its tikun will apply here also.
On occasion, it can happen that all three levels of soul will reincarnate together and become rectified. Then the Nefesh or Ruach of a righteous tzadik may impregnate him [as an ibur]. When he leaves this world he will ascend to the same level as the righteous tzadik that came to him as an ibur. In the World to Come they will actually be on the same level.
Although his own soul was not originally on the same level as that of the righteous tzadik, he has now merited ascending to that level forever.
Rabbi Shimon bar Yochai …said that in "that world" [the World to Come], both he and Rav Hamnuna Sabba would be shining together.
This is the inner meaning of what is written in the introduction to Zohar Bereishit (7a): Rabbi Shimon bar Yochai fell on his face and he saw Rav Hamnuna Sabba. He said that in "that world" [the World to Come], both he and Rav Hamnuna Sabba would be shining together.
Since the soul of Rav Hamnuna Sabba was within Rabbi Shimon bar Yochai as an ibur, they would be on the same level in the World to Come.
This is enough.
[Commentary by Shabtai Teicher.]