ADDING WIFI DEVICE TO PREEXISTING PREPAID ACCOUNT

You don’t.  A data only device will have its own separate prepaid account.  Data only and pay per use plans are not eligible for multi line.

The only multi line plans are the 4 phone plans with data included. ($35,$50,$65,$85) and the $30 monthly talk and text only plan.    https://www.att.com/att/prepaid-multiline-discount/

https://www.att.com/prepaid/plans.html

Not possible.   The multi line convenience is only for the monthly phone service. 

You would have to switch to postpaid to combine non phones on one payment