I want to buy an external charger, it has USB. Are there any adapters (including transformators, I think that might be needed, right?) so I can charge my MacBook with the external charger?
That is not possible.
The USB can not carry the Voltage (14 V) and Current (> 1 Amp) required to charge MBP.
If it is for the car, you can try this product http://www.amazon.com/Spal-Magsafe-Adapter-Charger-Notebook/dp/B009G1B22O
There're external batteries like this: http://www.amazon.com/HyperJuice-External-Battery-MacBook-Smartphones/dp/B009I5AZTY