A scheme can be extended and overridden by an add-on.
In order to extend a scheme, you should first create the directory schemas within the add-on directory (if it does not exist yet). In this directory, you can reproduce the core schemas directory structure to address a particular scheme. The scheme can be extended both by prepending and appending it with additional content. Name your files according to this pattern to extend the according scheme: <scheme_name>.[pre|post].php.
For example, you can extend the scheme block_manager/blocks by creating the schemas/block_manager directory in your add-on directory and putting the extension data in the file blocks.pre.php or blocks.post.php (or both).
You can see this technique in action in the Affiliate add-on , which extends some schemes, e.g. block_manager/blocks.php.?