Hello Ruben,
Although it is not mandatory to update source kernel, it is always recommended to update the source to a kernel which is more current and supported by SAP in case you run into any issue during export of source system. The system copy guide strongly recommends updating system copy tools such as R3load, R3ldctrl etc.
Using database specific method, you could potentially install the target with the same SAP kernel version and same version of database (MSSQL 2005) and later after copy upgrade to a more recent or supported version of MSSQL 2012, however export/import is much cleaner and recommended if your DB size is reasonable.
Regards,
Siddhesh