I am sure I read somewhere a few years ago of someone converting their Sora STi 8 speed to 9 speed.
I love my Sora and wouldn't swop it for even Dura Ace that said it would be really handy to have the extra sprocket to spread my gearing a little better. I may buy a set on ebay and have a go .
I believe Shimano may have made a marketing error with Sora . I think they weren't confident enough of uptake so intoduced it at the bottom of the range .