Activity pxGetOperatorDetails is an OOTB Final rule. You are not allowed to modify it. This activity will get operator details (like pyUserIdentifier, pyUserName, pyImageFileName, pxUpdateDateTime, pyEmailAddress) by OperatorId (param).
Can you write your own activity to get your required data (FirstName+LastName) from the tables?
1. Save as activity with different name in your application ruleset and use it wherever required.
2. If you want save with same name . Pega will not allow it. However we can do it by skipping validation on final rule while saving rule. Private edit pzCheckForFinalConflict (Rule-) activity , add step 1 as Exit-Activity and save. Now you can save activity with same name in your ruleset and modify accordingly.However this is not recommended at all. it leads to many regression issues in future. it will overrride all new functionality in future release.