Adding text to subject when calling group

[steve]

Hi,

In the alias file, I defined several users and with them defined a group
'groupX'. When creating a new message with 'm' and calling 'groupX', I
would like to add a fixed text to the subject line (only when the alias
'groupX' is called'). 

How could I do that? I failed to find that particular usecase on the
Net.

Thanks

Steve