I just created a simple plugin that adds option to export Roundcube contacts in CSV format. It will display format selection dialog when you use the Export button. The plugin source code is here.
Currently the code is very simple. In future the plugin could be extended with additional features like: localization of field labels, definition of fields list, selection of fields separator, new line character (Windows vs. Linux) or even output charset. Would be also nice if Roundcube core could import this CSV output without data loss.