The Famicom had one 'expansion slot' (which was not present on the American NES). Before NES was even released in America, a keyboard was released for the Famicom in Japan. The form of BASIC (NS-HUBASIC) that was included enabled users to play around with Nintendo graphics and sprites. Perhaps it's possible to find a copy of the manual, and have it translated?
The Educational Cartridge for the TV computer comes with GBASIC. I know very little about its capabilities, but I'm keen to run any routines you'd like. However, I find it incredible that a $12 computer could be a platform to teach BASIC (which can serve as a great introduction to computer programming.)
The Famicom system had a mechanism for saving BASIC programs, Music compositions, etc. This was the Famicom Data Recorder: a cassette player/recorder that plugged into the famicom, via 1/8 inch headphone jacks. Famicom Turbofile II by ASCII This is a "memory card" which was compatible with some of Ascii's Famicom games. The switch on the left selects which of four memory banks you want to use and the switch on the right is a "write protect". http://superfami.com/fc_turbofile.html
Great Descriptionhttp://www.old-computers.com/museum/computer.asp?c=517Famicom BASIChttp://nindb.classicgaming.gamespy.com/hvc-fb.shtml#releasesWikipedia Articlehttp://en.wikipedia.org/wiki/Family_BASIC_Keyboard