hi Steve,
Within a mains powered charger there is a stage of the circuit where the ac from the mains is transformed down to a lower voltage ac and then put through a bridge rectifier to turn it into dc. This dc power is then put through charging regulation circuits and used to charge the battery.
I have seen it done where solar panels were connected to the point in the circuit after the bridge rectifier, effectively injecting into the charger the dc power that would usually come from the mains vi the transformer and bridge rectifier. You have to put the right voltage into the circuit, but you could figure this out by dismantling the charger, switching it on and measuring the dc voltage that is present at the output of the bridge rectifier.
link of this being done on youtube by some americans:
http://www.youtube.com/watch?v=vk9GcG9EJzA
Alternatively use lead acid batteries which are much easier to charge using solar power.