Problem is that to properly apply the solder both surfaces have to be heated up to the point where the solder will melt.

As far as I have seen lead solder has a lower melting point than the lead free stuff so any leaded solder on the board will melt.

You _really_ _really_ are just far better off getting some desoldering braid and/or vacuum desoldering tool so you can remove most if not all of the lead solder then just apply nice fresh solder the the pads/vias/whatever.

Seriously. You cannot do this without exposure to lead. Now, if you have a solder paste.. perhaps. perhaps. It would not turn out pretty though.