As far as I can tell, this is simply not possible (and nor is it judicially ethical either). I would repeat for the zillionth time: this is NOT an appeal in the UK/US definition of the term. It is a normal trial, with a prosecution, defence and judicial panel.
The trial will therefore absolutely definitely proceed through all the normal phases of any criminal trial. We are currently in the evidence/testimony phase. This phase happens to be far shorter that would be usual, owing to the fact that all the evidence and testimony introduced in Massei's court is automatically used as the basis of the evidence/testimony in Hellmann's court. Hellmann can then consider requests from the prosecution/defence for additional evidence/testimony, and will grant the requests if he feels that the additional evidence/testimony will supplement that which is already available via Massei's court. This is what is currently happening in the case of the independent DNA report (together with Curatolo's recall to the stand, and the inmates' appearances).
In any trial, the evidence/testimony phase is completely separate from the argument phase. During the evidence/testimony phase, no party (prosecution, defence, victim's advocate, court) is allowed to argue for guilt or acquittal - the only questioning allowable is to establish the accuracy/truth/reliability/credibility of the evidence/testimony itself. It is only during the argument phase that the parties use the body of evidence/testimony as a basis for arguing for the guilt or non-guilt of the defendant(s).
In this appeal trial, exactly the same thing will happen. When the evidence/testimony phase finishes (which looks like happening at the end of the month, according to current rumours), the trial then moves into the argument phase. All parties will make arguments to the court, based on the entire body of evidence/testimony: i.e. all the evidence/testimony from the first trial, plus the additional evidence/testimony introduced into Hellmann's court. The prosecutors (plus, presumably, Maresca) will argue for guilt, and the defence will argue for reasonable doubt and therefore acquittal. The judicial panel will then deliberate, and will come to a verdict.
And that is how this trial will go. Remember that criminal charges in Italy are brought not by prosecutors but by courts, and I don't think that Hellmann would have the power to withdraw/dismiss the charges at this stage. The prosecutors will (rightly) have the opportunity to argue for guilt, and the defence will argue for acquittal. In my opinion, it's very highly likely that Knox and Sollecito will ultimately be acquitted at the end of this trial, but only after due process has been properly observed.