Sure, here are the completed sentences:
1. Officer Stanley didn't start the meeting **until everyone had arrived**. (BEFORE)
> Officer Stanley waited until everyone had arrived **before he started the meeting**.
2. After searching for hours, the volunteers **decided to take a break**. (BEEN)
> The volunteers had been searching for hours **before they decided to take a break**.
3. It was the first time they had used the **alarm system**. (NEVER)
> They had never used the alarm system **before**.
4. They didn't start the trial until all the **witnesses had arrived**. (BEFORE)
> They waited until all the witnesses had arrived **before they started the trial**.
5. The lawyer visited his client before he **went back to his office**. (UNTIL)
> The lawyer didn't go back to his office **until he had visited his client**.
6. After serving as a police officer for 16 **years, Kevin became an inspector**. (HAD)
> Kevin had served as a police officer for 16 years **before he became an inspector**.
I hope this is helpful! Let me know if you have any other questions.