Hi Chris,

It's a bit rough-n-ready, here's a quick code sample:

using umbraco.cms.businesslogic.datatype;
using umbraco.cms.businesslogic.web;
using umbraco.cms.businesslogic.propertytype;

DataTypeDefinition dataType1 = new DataTypeDefinition(1234);
// DataTypeDefinition dataType2 = new DataTypeDefinition(5678);

List<DocumentType> docTypes = DocumentType.GetAllAsList();

foreach (DocumentType docType in docTypes)
{
    int tab = docType.AddVirtualTab("New Tab");

    docType.AddPropertyType(dataType1, "New_Field_1", "New Field 1");
    docType.AddPropertyType(dataType1, "New_Field_2", "New Field 1");

    PropertyType pt1 = docType.getPropertyType("New_Field_1");
    PropertyType pt2 = docType.getPropertyType("New_Field_2");

    docType.SetTabOnPropertyType(pt1, tab);
    docType.SetTabOnPropertyType(pt2, tab);

    docType.Save();
}

First get the data-types for the properties (fields) that you want to add.  Then get a list of all the document-types and loop through them.

Add the new property-types to the document-type.  Now here's the PITA, the "AddPropertyType" is void, so doesn't return the new property-type, so we need to fetch it back from the database.  Once you've got the new property-types, then you can set which tab they belong to.

Save the document-type to trigger any associated events/hooks. (You can remove this if you like?)

I haven't tested this in any way... so use at your own risk! ;-)

Good luck, Lee.