I would guess it's not possible -- at a minimum, it would require soaking the film in something, which would tend to wash out the antihalation and sensitizng dyes, as well as requiring prolonged drying in the dark prior to rerolling. Remotely possible this might be practical for sheet film (if some means is found to reimplant the proper amount/type of sensitzing and antihalation dye), but beyond consideration for roll film.
The only potential I see is if the dopant can be applied in gaseous form, like forming gas used to hyper films (98% nitrogen, 2% hydrogen), but this seems unlikely (and in any case, formic acid, which would create formates, has a pretty distinct vinegary odor and is likely to absorb into backing paper and such, causing uneven sensitization.
More likely the doping has to occur at the point when the halide crystals are being formed and/or coaxed to grow in a particular crystal habit (cubic, tabular, or epitaxial instead of the native octohedral), which would make it impossible to apply aftermarket.