How to build an extractor?

Using SAP BW 2.1C and soon 3.0A we have to extract data from R/3 FI.
Standard extractor can´t be used, since data need to be enhanced and go throug various routines before data contains all information needed in BW. Delta upload mechanism is needed.
Where can information on
- building a new extractor (including delta mechanism)
- enhancing a standard extractor be found?

Technically, programming a function module data source would be a modification. There's also a way that allows you to stay close to the standard, especially if you extract data from standard tables:

In OLTP:
1) Find out which change document object points to the table.
2) Use the generic extractor to build a data source for _master data_ over the table.
3) Use the "enhance extract structure" feature in the BW img to add computed fields to the data source
4) Use the user exit for extraction to program your enhancement logic and compute additional fields
5) In data source maintenance, enter the change document object to delta-enable the data source

In BW:
7) Create an infoobject with attributes. (Key figures can be attributes, too.) Use concatenated keys or "Klammerung" to make the key fields identical to the key fields of your source table.
8) Create infopackages for initial load and delta load, and load data.

Now you need to load the data from your infoobject into a data target for transactional data (ODS or InfoCube).

In BW:
9) In infoobject maintenance, look up the view for master data for your new infoobject.
10) Use the generic extractor to create a generic datasource for transactional data out of this view.
11) Load data from this second generic extractor into your Cube/ODS.

As far as I know, BW 3.0 will allow loading data straight from OLTP _master_data datasources into BW ODS/Cubes, so as soon as you've upgraded, steps 7 - 11 will be obsolete.