Are you asking if you can add another LAG/ aggregated interface? Yes. If you already have:
set chassis aggregated-devices ethernet device-count 5 and you want to add another with
set chassis aggregated-devices ethernet device-count 6, the answer is yes. While you are at it, if you think may need another soon based on rojections you can set the count to 7 or 8. Then use them as needed. IThe command will not add a new statement, it will only modify the count number.