Your fitbit will fall apart before the battery is damaged. My wife is a CNA in a nursing home and when she charges hers, it's left on the charger for at least 24 hours. Other than the rubber encasing the device separating from the surface, hers works flawlessly even with 24h+ charges.
Also, im pretty sure the charging circuitry is inside the charger itself. If you ever plug an off brand charger into your phone or other mobile device, more often than not the device will tell you to use the charger provided with the device and it's on a "slow charge". If the circuitry were inside the device, I would imagine that the charger wouldn't matter as long as the power supply was correct. However, I am not an electronics expert.
I see no issues using the M18 power station to charge your fitbit, or any other mobile device for that matter.