The available positions for your modules are defined in the templates/your_template/index.php file inside your Joomla directory. You can create a custom position by adding it into this file.
For example, if you want to create a position named " custom", you should add the following code into the index.php file where you want the module to be displayed:
<?php if ($this->countModules('custom')) : ?>
<jdoc:include type="modules" name="custom" style="xhtml" />
<?php endif; ?>
Once you have done that, go to the Module manager, edit the module you want to display into this position and type manually custom into the "Position" field. Then, clear the cache of your Joomla application and check your website to see the new module position.