how to add datetime value and varchar value

hi
SELECT JobNo,VendorName,BillNo,BillDate,Remarks,DATEADD(day,Payment,BillDate) AS OrderPayDate from tablename

i am using this sql commnd.but i am getting the error
problem:
(1)dateadd(day,paymand,billdate)
paymand is varchar datttype
billdate is datetime

billdate paymand
22/4/2012 4
now i am adding billdate and paymand
i want result 26/4/2012
i am getting this error

Argument data type nvarchar(max) is invalid for argument 2 of dateadd function.