Don't do the join. Instead, start by loading the Payer and Exclude fields from the Excel sheet:
Payer as ExcludedPayer
From Excel ...
Where Exclude = 'Y'.
Then load your fact table:
if( Exists(ExcludedPayer,Payer), // if Payer already exists in the previous table
if( Payer = 'X' and Field2 = 'Z', 'N', 'Y'),
'N' ) as Exclude
... or some similar logic in the if-function.