Then that tool palette is linked to a tool palette in a tool catalog and you can not add tools to it, directly. If you have write authority to the folder in which the tool catalog is kept, you should be able to right click on the tool palette and choose Properties... from the context menu. At the top of the Palette Properties dialog, there should be a Refresh from: toggle, with the catalog to which the tool palette is linked indicated.
If you uncheck this toggle and select OK to close the dialog, you will then be able to add tools to that palette. Once you are done adding tools, open Content Browser, open the tool catalog in which the tool palette is stored (open the catalog, if it is not already open in Content Browser and navigate to the location where the tool palette is stored within the catalog (if necessary). In AutoCAD Architeture, right click and drag the palette and drop it into the Content Browser (if Content Browser is not "on top", hover over the Content Browser button in the Task Bar until it pops up to the top). It should ask if you want to update the existing copy - say yes.
Back in AutoCAD Architecture, edit the palette's properties again, and recheck the Refresh from toggle. The palette should reload from the catalog, with your newly added tools.