Using SQL Server Stored Procedure From Excel

I am a SQL Server DBA trying to use Excel to execute both read and write stored procedures from Excel.
I have 2 questions regarding this.

(1) I was able to fumble through some documentation online and put together a spreadsheet that can pull in data from SQL Server using a stored procedure. This stored procedure has a default parameter that defaults to NULL. I hope the reader knows what that means. Anyway, The spreadsheet is setup to read in the parameter value from a cell. It works fine when I provide a specific value for the parameter, but not sure what I have to type in to the cell for it be interpreted as NULL. I tried leaving the cell blank, but that doesn't seem to work.

(2) How do you execute a stored procedure that writes data back to SQL Server. Can someone direct me to a tutorial?