This request (sharing entities between projects and maintaining a ''pool'' or ''template'' of resources) is WHY the ''ALM-level'' license exists for Quality Center.
With the ALM license (instead of ''enterprise''), you can setup a ''baseline'' (with content like tests and requiements) and ''publish'' various entities across projects. Some companies maintain a ''master'' library of these entities and ''publish'' parts out to various smaller projects.
Another feature of ALM is the concept of having a ''template'' project containing customizations (custom workflow and fields) and initial structure (folders), then this template can be used to create projects with the same structure and later push additional customizations down.
If you don't have the ALM licence, you are left with the old multiple IE instances showing different QC projects (at same version like both running QC12.55), then doing copy to clipboard, then paste into target project. Be careful not to grab too large of a ''chunk'' (of the test tree for example) or you may hit a maximum items copy limit or memory limit.
As for copying cusotmizations, you could have a ''base'' project that contains all of the basic starting point for your projects (like custom fields and workflow pre-defined and any starting folder structures), then when you need a new project, create one based on this one. After that, make sure to do any new customization additions to the ''base'' AND all other projects at the same time.
Be careful with adding custom fields -- each one gets the next consecutive integer (e.g. BG_USER_02, BG_USER_03, etc.). If the admin(s) add(s) random additional custom fields willy-nilly to the projects, different numbers get assigned to the SAME named field in different projects. This is not a big deal... UNLESS you want to combine data from different projects in a REPORT ... OR Copy it from one project to the other.
Yet another feature of ALM license is ''cross-project reporting'', where you can grab data from more than one project (assuming the data structure is the same, or almost the same).