Assume I have a question/problem. I search through existing questions & find that a particular question & the corresponding answer pretty much match what I needed but either a small part of my question is not answered or I need to ask for some clarifications. If the original question is closed because it got answered, then what is the protocol - Should I ask a new question & reference the original or - Is there a way I can reopen the question (asked by someone else) and add extra questions I have?
In one word: comments. Earn enough reputation to have the privilege to post comments (50 in Stack Overflow) then on the most suitable answer post comment asking for clarifications. I get such comments frequently, and do my best to address them.
Question is not closed because it get answers, so I guess you mean "marked as solved" by having an accepted answer - in such case people can still post answers, it's not closed at all.
Anyhow, if your problem is just related to that other question and you need different solution then posting new question, possibly referring to the other question, is usually valid thing to do.