Sorry to disappoint you, but no, you can only create Structure groups and pages withing a Structure group. Page will always have url based on its structure group path.
If you want some pages not seen in certain publications, you might consider organizing them via Blueprint, so that they are seen in some publications, and not in others.
Based on your comment, there is something that you MIGHT be able to use. You can use blueprint to organize your website. How to achieve this? Basically, you can split your website into multiple websites (Publications). What this means is that you can set URL per publication, and all SG and pages within that publication have one URL.
For example, your Abstract SG will become Abstract publication with following url:
/root/AbstractURL1/. Second publication will have url:
This can help you have better organization of pages, but they ARE SPLIT INTO SEPARATE PUBLICATIONS, with separate content etc... Honestly, this kind of looks like overkill for your request, but this can be done.