If the agenda is locked, but you wish to add additional documents, you can add them as appendix items from the queue folder – in the same way as you normally add agenda items from this folder.
Right-click on an agenda item in the Queue folder and select Move to meeting from the context menu.

Note that there is a difference: when selecting a meeting, it will say Add as appendix, and under the meeting's properties, the items will be under the "Appendix" tab.

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Tip:
To lock the appendix, you must navigate to the meeting, right-click on it, and select Lock sppendix agenda from the context menu or toolbar.

The confirmation dialog will be displayed:

Here you have the following options: