First of all the translator for DB2 is already available in the Teiid, you do not need custom translator. The ExecutionContext has a method called "getSubject()" that will return the subject for the logged in user into Teiid not the data source.
If you are looking for custom security at the "data source" based on the logged in user etc. please read about Login Modules in the Admin Guide. If you want you can create your own custom login module.