If you want to add more extensions you can use the appbrowser to download and install them. If you want a directory to put them in, you can create a directory on your hard drive or usb drive. If the directory is named tce it will be automatically found, but you can name it what you want. If you name if tcz, then use the boot code "tinycore tce=hda2/tcz" if you have a directory called tcz on your /mnt/hda2 partition. I hope I understood correctly.
This topic would probably belong in the TCE Q & A Forum.