[Help-gnu-radius] How to create missing attribute ?

[Vlad -]

Hello to all,

Some of my NAS'es are sending accounting requests without User-Name 
attribute.Gnu-radius must forward this packet to the next radius. 

Can't guess how to create missing attribute (IF "it really missing") before 
packet will be processed. The only thing that was done with rewrite - inserting 
record to "calls" table with User-Name="NONE".
But proxying for this packet doesn't work. 

1. Is it possible to verify attribute existence and if it  missing create it 
with some rules?

2. How to check existance attribute in rewriting function? 
What value will be returned by expression 

if (%[Attribute])

if Attribute is not exist?

Thank you,
Vlad