Need to convert a Case statement into an expression in SSIS

I had to take a vendors sp and it is a mess.  So I am calling their sp from within my SQL package.  I have to pass in a start date and end date.  The day of the week dictates what the dates will be.  The 2 different ones are Monday which grabs previous Friday's date.  Tuesday grabs Saturday, Sunday, and Monday.  All the rest of them just pull the previous day.  I had this set up in a new stored proc but that isn't going to work.  So I want to add the dates as variables in my SSIS package and pass them into the vendors SP.  I have that all working and pulling my data based on just using the previous days data.  But now I need to get it set up to work like I had mentioned before based on day of week.  Not sure how to do this in Expression Builder.

This is what I did in the stored proc:

SET @begdate = (SELECT 
CASE 
WHEN datename(dw, getdate()) = 'Monday' THEN 
CONVERT(VARCHAR(10), dateadd(d, -((datepart(weekday, getdate()) + 1 + @@DATEFIRST) % 7), getdate()), 110)
WHEN datename(dw, getdate()) = 'Tuesday' THEN 
CONVERT(VARCHAR(10), dateadd(day,datediff(day,3,GETDATE()),0), 110)
WHEN datename(dw, getdate()) = 'Wednesday' THEN 
CONVERT(VARCHAR(10), dateadd(day,datediff(day,1,GETDATE()),0), 110)
WHEN datename(dw, getdate()) = 'Thursday' THEN 
CONVERT(VARCHAR(10), dateadd(day,datediff(day,1,GETDATE()),0), 110)
WHEN datename(dw, getdate()) = 'Friday' THEN 
CONVERT(VARCHAR(10), dateadd(day,datediff(day,1,GETDATE()),0), 110)
END)

Help please.