The original charging cable says 4,6V DC and 90mA, but i have a power bank with at least DC 5V / 1A and i bought an USB cable to charge the 3DS, is it safe to charge it with that power?

The 3DS like any properly designed device will only draw what it needs for current. That means that charging with 5V 1A or 5V 50A would charge at the same rate and not damage the 3DS. Charging at too low of a current like 100mA for example would charge too slow and not charge the console.