As an example, see this question.
This is probably a bad example, but should I edit the question to include a tag like timespan, because it is relevant to the solution but did not get mentioned by the user who posted the question?
In general, if a question does not include a tag I consider relevant to answering the question, should I edit the question to include it, maybe to make users following that tag visit the question and post an answer?
If you are sure that the tag is relevant to the post, of course you can do it. You pretty much gave an answer yourself when you said :
maybe to make users following that tag visit the question and post an answer
since that's the whole point of the tag system. Sometimes, the question is mistagged by new users and comments or further reading of the question will make you think you should retag it. It is the same principle for given answers. Think of it that way :
When you see the question and it seems to you there is a missing tag, you add it ?
Well the answerer to that question knew that it was about this tag since he talked about it in his answer so he could (maybe should) have changed it by looking at the question.
So yes. Just make sure it's relevant.
