You can't use 'sdelayer' for this purpose. The sole purpose of that
utility is to manipulate layers (the geometry column), and the '-o add'
option only adds geometry columns (max one per table).
If you are working with "feature classes" (the ArcGIS name for layers)
using Desktop, then you would be better served by using ArcGIS to add the column.
This will prevent issues that would be caused by using SQL to add
unsupported column types, or failing to add the corresponding column
to the version and/or archive history tables (if present). You would,
of course, need to be connected as the table owner, and the table must
not be locked by an application, and the Desktop license level can't be
ArcView (now known as Basic).
If you do use SQL to modify a table registered with ArcSDE, it is important
to execute 'sdetable -o describe' on the altered table, as the owner, to
make sure the SDE.COLUMN_REGISTRY table is updated. Altering a table
which has been registered with the geodatabase and participates in
versioning or history or other ArcGIS behaviors using other than ArcGIS
is not recommended.