You can achieve that using Relations and ExpressionToApply.
I think your sql can be rewritten as:
UPDATE T1
SET F1 = F2
FROM T2
INNER JOIN T2 ON T1.PK=T2.PK
Here is an Northwind example. The example updates Orders.ShipAddress with Customer.Address related.
// values to update
OrdersEntity newOrderValues = new OrdersEntity();
newOrderValues.Fields["ShipAddress"].ExpressionToApply = CustomersFields.Address + "...";
// set filter and relations to use
IRelationPredicateBucket filter = new RelationPredicateBucket();
filter.Relations.Add(OrdersEntity.Relations.CustomersEntityUsingCustomerId);
// update entities
using (DataAccessAdapter adapter = new DataAccessAdapter())
{
adapter.UpdateEntitiesDirectly(newOrderValues, filter);
}
So, adapt the concept to your case and let us know if everything is ok