Can a signature in ballpoint pen ink be darkened after it's started to fade on a Broadway poster?

I collect Stage Play Posters from Broadway and London shows. I went to a few shows unprepared, and I foolishly had cast members sign the posters in ballpoint pen. Over the past 6 or 7 years, the ink has already begun to fade. I am hoping there is a way to either preserve what is left and stop it from fading, or some way to "resurrect" what was there and darken the signatures. Anyone?