Taking on an additional job
If the basis for legal residence authorises a foreigner to undertake work and the foreigner is among persons exempt from the obligation to have a work permit, to take on an additional job it is sufficient to execute a proper contract with the employer.
In other cases it will be necessary to obtain an additional work permit to take on a job. Put simply, new job = new permit. Only after such a permit is obtained, the contract can be executed and a foreigner can undertake work.