According to the Chart of Callings found at the end of Handbook 2: 19, everyone attached to the bishopric including clerks, assistant clerks, and executive secretary is listed as "Approved By: Stake presidency and high council" and "Called and Set Apart By: Stake president or an assigned counselor or high councilor".
While the calling of assistant executive secretary isn't a standard calling, I'm not sure by what logic one would assume that the ward could do it without the stake's blessing.
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