If using Nlite and added the Sata drivers during the slipstream, but chose not to install them at the OS installation (using the hard drive IDE setup), will one be able to switch to the Sata drivers after install is finished? The way it is now, one must do the F6 thing to install the Sata drivers before installing XP. If you didn't do that, installed XP and then decided to use Sata drivers, it was a difficult process to install them after installation.

Yes, you have to replace the "Standard Dual Channel PCI IDE Controller", which is managing your S-ATA hdd's in IDE mode, by the Serial ATA Controller (Intel or Silicon Image) where your S-ATA hdd's are connected.
Open your Device Manager and look into the IDE ATA/ATAPI Controllers section. Right click onto the Controller you want toreplace and do a driver update by loading your specific Sata driver.
After the reboot your S-ATA hdd's will be managed by the third party S-ATA Controller.

I think there might be a little more to it than that. With my motherboard, there is a setting in the BIOS something like "configure as SATA or IDE". If during a normal XP install, the SATA drivers were not installed (via the F6) and the BIOS was configured for IDE, if later trying to go back to BIOS to set to run as SATA, XP would not boot up because the drivers were not there before XP loaded. The SATA drivers also included having to install the Intel Matrix storage manager program.

You are right, I misunderstood your question.
If you want to use the AHCI features of your Intel S-ATA Controllers, you should either reinstall XP with integrated Intel S-ATA drivers (look here or do a Google search for a manual update method to the AHCI mode from within Windows XP.