Charger for the own created battery pack

The pack will have to somehow be modified to charge the cells in parallel (right?) while powering the device in series.

Yep. A relay is an easy way to do this.

See http://www.batteryuniversity.com/ for the specific type of battery you're using.

To recharge lead-acid batteries, for instance, you start out with a trickle charge to make sure the battery doesn't have any shorted cells, etc.

When it's up to a certain threshold, you do a constant-current charge. The maximum current is specified by the battery manufacturer. (You measure the current with a small current-sensing resistor in series with the battery. A 0.1 ohm resistor with 100 mV across it has a current of 1 A going through it, for instance.)

After the constant current stage, you apply a large constant voltage, and then when that stops taking current, you drop back down to a float charge.

Here are instructions for nickel batteries.

The charging voltage will be 1.4 V -1.6 V per cell. They must be charged at constant current, not 1.2 V per cell.