Add field to CONTENT mail

I want to add another field in [CONTENT] mail.. How can I do this? In which file can I find settings for [CONTENT] function?

example... my mail say:

User : nicocliente
User name: : nicocliente

address
name:: nicocliente
E-Mail: blablabla
name: Nico
surname: Dia
address 1: Via dssdf
CAP: 70124
city: Bari
town: Italy
State: Bari
Telephone: 08854545454

I want to add field "user type".... Where Can I insert this field?

Thanks