Oh ok, thanks for the reply. I looked through the manual but didn't really see anything. So what you mean is I can make a function something like myConstant() and use that in my expressions to get that number, right?... What about creating something like "_myNum" to use in expressions as with other CAS-model constants? Is that possible?

The manual I read is here: http://www.seg.etsmtl.ca/nspire/documentation/TI-Nspire_CX_Handheld_GettingStarted_EN.pdf