First, how oxidized are these solder balls? Will the take solder using your routine flux / paste?
Second, on restoring solderability, the three basic methods you can use to restore solderability on components are: * Highly active fluxes in a retinning operation. * Surface stripping chemistry and then a standard retinning operation. * Electrochemical conversion chemistry and then a standard retinning operation.
Keep in mind that none of the three basic methods work on everything and all are highly dependent on just what/amount of finish remains on the component lead and the amount of oxidation/corrosion that is on the component lead.
Highly active fluxes are good, but residues from poor removal methods can cause problems later on in the assembly.
Surface stripping chemistries (eg, Kester SolderNu) work well, but require protection to prevent the stripping chemistry from attacking component bodies. Stripping chemistries are specific to the surface (eg, copper, nickel, or alloy 42, etc.) to be stripped.
Electrochemical conversion is great, but level of cleanliness and some surface materials (eg, iron) can contaminate the electrochemical cell. ROSA cathode grids have an iron tolerance on the that requires periodic maintenance schedule attention.