After the fermentation, I tasted it to see how it was. It was a little weak. I added more malt and yeast, but it didn't ferment. Is it ok to bottle, or do I have to do something to it to kill the yeast?
First off, welcome to the HBT forum!
I wouldn't bottle unless I was sure it had finished and, particularly given the method you used, I'd want a hydrometer measurement to know for sure. If you don't have one, get with a local homebrew shop or order one on line; it won't hurt for your beer to stay in the fermenter for a while longer.
Some other questions (including from Revvy):
1 - how long has it been since you first brewed and added the yeast?
2 - how long has it been since you had the second batch of malt extract and yeast?
3 - at what temperature(s) has the fermenter been?
As to tasting "weak" are you talking alcohol-wise or taste/mouth-feel? Aging helps the second one considerably. I always taste the samples (well, actually, drink the samples) I take and you could describe the taste as weak, flavor-wise, before it has had some time to age.
What was the stout recipe?
Rick