You can set the beginning of the stack from 0 to 0xff and of course you can write into the stack area even with C.
When something within your program goes wrong and you'd like to get some help you should be a bit more specific:
Which PSoC1 are you using, what are the settings etc.
If you can, use Designer -> File -> Archive Project and upload (best is to use ms internet explorer, chrome doesn't work) the resulting archive so that we all can have a loook at.
My post was meant more as a complaint than a question.
I got it working by setting the pre-set value address (end-of-stack address) as 0xF7 (accounting for the 8 bytes used by the E2Prom library).