The following VBScript code can be used inside a "Run VBScript" Action, in order to open a password-protected Excel document.
Set xl = CreateObject("Excel.Application")
Set wb = xl.Workbooks.Open("%Excel%",,,,"%PasswordtoOpen%")
xl.Application.Visible = True
%Excel% -> Variable that contains the Excel file path
%PasswordtoOpen% -> Variable that contains the Password
After opening the Excel, the "Attach to Running Excel" Action can be used in order to go on with the Excel processing using the Excel Actions.