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In data.pak, you find the list of animations for the model of the desired creature (AnimSet) and add a new element there with the name "cast" and a link to one of the existing animations. You can also do this through the editor: select, for example, an archer and highlight it; in the property tree, in the "shared" line, click on the ellipsis, then do the same in the "AnimSet" line, and then proceed as described above.