Is it possible to spell a word to Siri? For instance, if I ask for the stock price of a certain company, she doesn't always understand the company name.
You can say
no caps on and
no space on and then spell a word, letter by letter, followed by
no caps off and
no space off and Siri will understand that you spelt a word out for it to interpret.
You can see this in practice in this video: https://www.youtube.com/watch?v=xMNLV-qeou4
The context of the video is spelling words in Notes.app, so it's not clear if this works in all Siri contexts.
It doesn't work reliably but there are a few commands for spelling things out in Siri. Here's an article about it: http://www.tuaw.com/2011/10/20/how-to-type-comma-using-siri/
It sounds like some Dragon Dictate commands work.
I'm afraid that's not possible. You can't force Siri. At least, not yet.