How to update SharePoint Content Type

A simple way is to do it in the feature receiver, get a reference to the SPContentType, add the field as XML (SPContentType.Fields.AddFieldAsXml) and then update pushing the changes to child content types

It is really tricky to modify content types on a site that already exists. My preferred method involves having a solution for all the content types and re-deploying that solution when changes are made.

So your letter template content type has a number of fields. The data for these fields is stored in an external database? sounds like a job for the Business Data Catalog (MOSS feature, not WSS).

Another possibility is an Item Event Receiver that connects to the database and retrieves the data. This is definitely not the preferred way of handling this scenario, but may be quicker to implement.

hth,

jt