Doing this would make it so that nobody is admin on any maps that don't have an amx_addadmin in the map-specific config file which seems like a strange thing to do. The OP said he wanted to add admins not replace/define all admins on a per-map basis.
FYI, the OnAutoConfigsBuffered forward does not exist in the currently released version of AMX Mod X (v1.8.2) so you should specify that you need 1.9.0.
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