From LimeSurvey Manual
Here is a simple solution to create question pools for LimeSurvey.
It will require access to the backend database. I'm not going to explain how to do this, as if you don;t understand it, then you shouldn't really do it. Hopefully an option will appear in a new version.
Basically 3 tables exist that we care about
lime_surveys - id field = sid
lime_groups - id field = gid
lime_questions id field = qid
In order to show the questions correctly the id field from the lime_groups table exists in the lime_questions table. This then is used to create the parent/child relationship.
However, if you want to create a pool of questions all you need to do is to amend the gid field in the LIME_QUESTIONS table to a value that doesn't exist in the lime_groups table. I would use 0 or a something silly like 10000. Try this with one question and you will notice it doesnt appear in the question list.
It's a crude workaround, but allows you to create as many questions as you want, without having them visible.
This could also be applied for groups as well just by using the sid field in the lime_groups table.