Increase of yeast won't change the likelihood of bottle bombs. If you do add extra yeast just open the lid and throw it in there, no need to stir it or anything. Use all of it. However, it may be better to let it go for a bit first and see if it ferments out properly. If it stalls you can throw more yeast at it if needed, but if it doesn't then you can save that yeast for another brew. B)