IF IS NULL then SINGLE else keep the value of the field
Try:

NZ(MaritalStatus,"SINGLE")



  From Nz Function in the documentation:
  
  You can use the Nz function to return zero, a zero-length string (" "), or another specified value when a Variant is Null. For example, you can use this function to convert a Null value to another value and prevent it from propagating through an expression.


Or you can use:

IIF(IsNull(MaritalStatus),"SINGLE",MaritalStatus)