I've added some custom enumeration strings to resolution values.

In custom_constant_inc.php:

resolution

define( 'DONE',             11 );
define( 'UNABLE_TO',            12 );
define( 'NO_RESOURCES',         13 );

In config_defaults_inc.php:
$g_resolution_enum_string = '11:done,12:unable_to,13:no_resources';

And appropriate values in custom_strings_inc.php.

Nevertheless, when new bug is reported, Mantis automatically assign it a value of 10, which corresponds to "open" in default settings. As far as the Mantis manual goes, this cannot be changed with Mantis configuration files - the value is possibly hardcoded. User should be allowed to change this value, preferably through variable in config_defaults_inc.php.