See my comments above. Yes, they can be rebuilt, the extent of the rebuild will be dictacted by the damage the turbo may have suffered.
The major question concerning rebuild is the fatigue life remaining in the components as the turbo grows and relaxes due to centrifugal forces on the wheels (low cycle fatigue), and the blade motion imparted by the exhaust gases going by the tongue in the volute and the impeller at the end of the diffuser where there is a tongue as well. Fatigue life is also impacted by any (internal) casting flaws. External flaws can be found by Zyglo, and internal be x-ray; doing the latter may be a cost trade off by the turbo manufacturer based on confidence in their supplier.
Pratically speaking, all the major turbo manufacturers offer rebuilt turbos on an exchange basis and independent shops offer rebuilds (may get your turbo returned) as well, so I wouldn't be too concerned about having a turbo rebuilt. Obviously, YMMV
John C