Posting here to not leave the question unanswered: this seems to be exactly what Community Wiki (CW) posts are for. See What are "Community Wiki" posts?.
Note that you cannot convert questions into CW after asking them. Such conversion can only be enacted by a moderator (and would obviously only happen in specific circumstances in which such a change would be deemed as useful/necessary). Personally, I think situations in which this is warranted are quite rare (as in, I cannot think of any that occurred in this site).
Answers can be converted into CW by the owner or moderators. This might be useful if you want to stress that the answer should be thought of as "collectively owned", or as a work in progress that others can contribute to and improve. But then again, I think there is no real need for this in most circumstances. It might make sense for the situation which you are thinking about: have an answer which is based on the state of the art in some field, which can be more easily updated when new things happen in the future, but I don't think it would be particularly appropriate to ask other people to convert their answer into CW. After all, doing so you forgo reputation from future upvotes, which might not be fair given the work one put into writing the answer.