Sorry about the account deletion; the forum adminstrator probably thought yours was a spam account...
Harvester2 implements its various schema as plugins; if you're familiar with PHP, see plugins/schemas/dc, for example. (This is the Dublin Core plugin.) If you'd like to implement a new schema, this is probably the way to begin -- but keep in mind that you'll also have to have a data source that supports this schema.
If you're just looking to extend Dublin Core, there's an example plugin that does this as well. See plugins/generic/pkpdc for more information.
The Harvester2 technical reference describes many of the concepts involved. It's available at http://pkp.sfu.ca/harvester2/TechnicalReference.pdf
Open Journal Systems Team