Charging packs all at once is problematic. Without the ability to monitor each cell, charging is prone to getting the cells way out of balance. This is because if you charge the whole pack as a unit, the cell that's the most discharged (even if only a tiny bit) will be the last one to take a charge, and the fastest to discharge when you put it under load. Over multiple charge cycles, this only gets worse.
On well designed power systems, if charging a whole pack there's a wire going to each cell and a circuit to monitor that cell's voltage, and if any cell's voltage goes over that of the other cells, the circuit drops a resistor across that cell to shunt charge power around it and into the other cells.

You can charge as a pack, but you can't fast charge. If you just trickle charge it, it would be fine as long as you don't leave it on there too long. I'd shoot for an 8 hour charge time if not longer. With nominal 2100 mah cells, there will be some losses, say over 8 hours you need to pass 3000 mah of charge current through those cells. 3000 mah over 8 hours = 375 ma charge current. Find a 7.5 or 9 volt adapter (charge voltage must be higher than nominal pack voltage), a 9v battery clip, and wire them in series with a resistor. You want 375 mah at 7.5 volts, that's 7.5/0.375 = 20 ohms, 7.5*0375 = 2.8, so find a 20 ohm, 3 watt (probably 5 watt would be easier to find) resistor.

It can be hard finding just one resistor in consumer sources. Amazon has them but $8 for 10 of them.

Still even just going to Amazon and throwing away 9 of the resistors, you could easily toss this together in a few minutes for < $20.