yep in early high late .txt

there's lines of the nature

Add_resource ID_WESSEX Iron Gold Silver
Add_trade_item Wales Sheep Strange_looking_women Rugby

just add in under the correct area.

Probably best making your own copy of early.txt and calling it somthing different and changing the name at the beggining of the text file so you get it as a whole new campaign. Just incase you break something.