There is no such thing as a middle join.  The question you saw was a trick question where the word "middle" becomes an alias for one of the tables.

I thought this was a very amusing trick question. 

It makes me laught to see how seriously some people seem to take the QOD, which is supposed to be a...

Here is a simple example

Declare @sql nvarchar(100)

SET @sql = 'SELECT * FROM <mytable>'

EXEC sp_executesql @sql

If I understand you correctly, you could use

-- this is the calling procedure

create procedure test1

as

Declare @give_param varchar(100)

Declare @a int

Declare @b-2 char(10)

Set @a = 10

Set @b-2 = 'spam'

Exec test @give_param out, @a,...