"this is automatically populated ..." when the customer's email doesn't exist/is invalid in your customer list. If you assign a CustomerID, it wins.
But, if that isn't good enough:
The customer information GUI through OTRS relies on the listing of Map for CustomerUser for the Customer Source chosen on the right hand side to look up the customer.
If your CustomerUser source is the CRM itself (why is this not a good idea?), you can Map any available fields.
If the CustomerUser is local/otrs database and you need a new field not referenced, you will need to extend the schema (alter table) to include the field, then reference that field in the Map.
If the CustomerUser source is, for instance, LDAP/Active Directory, you can use any relevant attribute in the Map.
The Map has entries that look like this:
# var, frontend, storage, shown (1=always,2=lite), required, storage-type, http-link, readonly, http-link-target
[ 'UserSalutation', 'Salutation', 'salutation', 1, 0, 'var', '', 0 ],
"UserSalutation" (UserMyField) is how OTRS will reference the field in Replies, Responses, etc.
"Salutation" (My Label) is what label displays in a form for this field.
'salutation' (my_field) is the fieldname or attribute for the back end/database/ldap (key)
1 is for when it is shown (almost always 1)
0 is whether the field is required
'var' is an arbitrary value that, as long as it's not "int" means "string"
'' (double apostrophes) is a field that holds a URL that encapsulates this field (see the link)
0 is whether the field is read-only (This can't be used to override the readonly of the source backend.)
not shown, but the http-link-target is if you want to use, for instance, "_blank" to pop up the link.