While it is possible for the Federal government to charge and convict someone for an act of which they've been acquitted of a crime in state court (the Rodney King case is the most notorious example) this seems to be a stretch in the Zimmerman case. The vehicle the racial grievance industry is proposing is the Shepard-Byrd Act. One problem is, both the crime alleged (willfully causing body injury resulting in death) and the motivation (because of color) seem to be already covered by the original second-degree murder charge, which required the defendant have acted out of enmity to the victim.
Most worrying, though, has been the fishing expedition by the Department of Justice to try to show that Zimmerman was racially prejudiced. Even though evidence presented at trial strongly indicated that Zimmerman was unprejudiced, the DoJ have gone so far as to set up a snitch line using which 'concerned citizens' can email evidence of Zimmerman's race hatred. Bill of attainder, anyone? If this works, Zimmerman is in jeopardy if he can be proven guilty of past thoughtcrime indicating racism.
This is perhaps the most insidious effect of hate crime laws. Racist speech, however much one might deplore it, is protected speech. If however it can be collected post facto and used to provide the basis for a crime carrying a life-sentence, how protected is it?
Meanwhile 'Justice for Trayvon' supporters are out committing overt hate crimes, completely ignored by the US Department of Justice.