Generally plugin code modification is needed for that:
1) add new capability to WordPress capabilities list;
2) in key places of plugin code check if user has needed capability and permit or not permit some actions.
If you not intend to change plugin code yourself may be to ask plugin author about new functionality will be better decision.
Abot user levels:
In short, User Levels were introduced in WordPress 1.5, they were replaced by Roles and Capabilities in WordPress 2.0, and were finaly deprecated in WordPress 3.0.
Prior to version 2.0, WordPress used a user User Levels system. This was replaced in version 2.0 with the much improved and more extensible Roles and Capabilities system you see today. To maintain backwards compatibility with plugins that still use the user levels system (although this is very much discouraged), the default Roles in WordPress also include Capabilities that correspond to these levels. User Levels were finaly deprecated in version 3.0.
More details at