I think its possible, it will require several steps but none of them are too complicated.
Firstly, use Power Query to create a new table with the email addresses of the managers and an additional line for "Show My Employees", something like
= List.Combine( { { "Show My Employees" }, List.Sort( List.Distinct( Employees[Manager] ) ) } )
where Employees is the table with the existing data, and Manager is the column holding the managers email addresses.
Turn the list into a table, and then add an index column. It should have the "Show my employees" line first and then a sorted list of all the email addresses. Call the new table e.g. "Manager for slicer" and load it into the data model.
Set the sort order of the email address column to be the index column.
Make sure that Power BI hasn't automatically created any relationships from this table to anything else. You want it to be disconnected.
Use the new table in the slicer instead of the existing email address.
Create a calculation group with a calculation item like
Include show my employees =
VAR CurrentSelection =
SELECTEDVALUE ( 'Manager for slicer'[Email] )
VAR Result =
IF (
CurrentSelection = "Show my employees",
CALCULATE (
SELECTEDMEASURE (),
TREATAS ( { USERPRINCIPALNAME () }, Employees[Manager] )
),
CALCULATE (
SELECTEDMEASURE (),
TREATAS ( VALUES ( 'Manager for slicer'[Email] ), Employees[Manager] )
)
)
RETURN
Result
Apply the calculation group & item to the visuals you need as a filter. That should show the manager's employees when they choose "Show my employees".
