This is a bit late so you probably already have an answer, but FWIW: the mAh rating describes the capacity of the battery - nothing to do with the actual current it supplies: that is dependent on the load. In this case, the 4.5V battery's 600mAh rating says it will supply 600mA for one hour, or 60mA for 10 hours, or 6mA for 100 hours etc. If the voltage is correct for the coils, then you do not need to add any resistance.
Originally Posted by glbeas
3 x AA batteries will supply a nominal 4.5V so that seems fine (rechargeable cells have a lower voltage - typically 1.2V each - but may still work depending on the internal circuitry). I assume that you are simply going to solder leads from the external battery to the existing battery terminals on the shutter? If so, that's all you need to do: you are simply supplying the voltage it is expecting - just make 100% sure you have the polarity correct.