As a general rule, if your provider can’t actually tell you configuration instructions for Asterisk, you may want to think about a more ‘Asterisk Friendly’ provider because this could be only the beginnings of your problems with them…
If I’m not mistaken, the From header can be changed by setting the ‘fromuser’ setting in the outbound trunk.
I don’t think the P-Preferred-Indentiy is standard, especially in that format. The two most common ways to assert your identity would be to set the ‘sendrpid’ to either yes (or in 1.8 to pai if that is the format the carrier wanted it). If they need something special, unless there is a setting I’m not aware of, you will have to create the header yourself.
If you need to do it custom, then see the other active thread in the form about someone wanting to add a diversion header, the same principle would apply but with your desired header.