Can I use impdp FULL=Y for an Oracle database charset conversion?
On export the documentation states:

A full export does not, by default, export system schemas that contain Oracle-managed data 
and metadata. Examples of system schemas that are not exported by default include 
SYS, ORDSYS, and MDSYS. 


If you want to be absolutely sure I would switch to a schema-mode import instead. You can still use the full export. That way you specify what schemas you want to import.

Regards,
Daniel