From Another Report:
VIII - When the Heartless are defeated, what becomes of the stolen hearts? Also, when members of Organization XIII and other Nobodies are defeated, do they return to their original form?
When Heartless are defeated, essentially the owner’s hearts are rejoined with their once extinguished body, whichever world it may be on. As for the whereabouts of hearts in KHII that turn up, this time they remarkably went to the Organization. However, in the rare case that the body changed into a Nobody and there is no container for the heart, it resorts to a state of suspension.
In the case of a Nobody being defeated, it’s a little more complicated. If the above mentioned hearts are liberated, they return to their original form. However, if the heart is still stolen by the Heartless, the Nobody’s body is swallowed by Darkness. If somewhere in the world their hearts are taken back, perhaps they might be able to return to their original human form.
So, all of the Organization members have a chance to return to their Others.
As to whether they knew about it, it seems like the kind of thing that might not be discovered very easily. After all, testing it would require being able to know the identity of Heartless, Nobody, and Somebody - and the willingness to sacrifice the Nobody for the sake of testing. Since Dusks seem to know no more of their identity than Shadows, very few Heartless are distinct enough to recognize, and those Nobodies/Heartless with known identities were generally the most valuable ones, it's likely that the Organization never had the chance to find out about that method of regaining a heart, and assumed that dying was a bad thing.