A lot depends on what is limiting its output to 250mA. If its just that the transformer inside it has that limit, there could be a certain amount of luck involved. The output windings will generate heat, depending on how much work they are doing - if the transformer can get rid of the heat faster than it is generated, no problem. If the transformer gets too hot, the insulation starts to degrade and will eventually fail.
Your arrangement will probably work OK, but it might need a bit longer than the charge time you are using now.