There is an example of adding a new access level is shown in http://manual.mantisbt.org/manual.customizing.mantis.enumerations.php Does this cover your needs?

I'd prefer to be able to change it direclty in the gui and not having to change any .php files as these might change in comming releases.

Adding new states as described in the manual does not require changing any files that will be overwritten in the next release.

Adding this via a GUI is difficult because of the language translations. We need to capture the new title in multiple languages, and this is just not set up in the tool now.

Thank you for taking the time to report a problem with mantis.

Since this problem report was originally made, a number of releases have occured. Additionally no recent feedback has been received on this issue.
It appears that this issue has either been fixed, or may not be a relevant report for the current release.

Unfortunately you are not using the latest version and the problem might already be fixed. Please download the latest release from http://www.mantisbt.org/download.php [^] [^]

If you are able to reproduce this bug in the current release, or have some more information on how this feature could be improved in the current release. Please either change the mantis version on this bug report
to the version you tested and change the status back to "Open", or open a new issue report with more information.

Again, thank you for your continued support and report.