I think you can add the display modes to /etc/X11/xorg.conf.

If you don't have a xorg.conf, then you can use the following as a basis. You need to replace the entries with the names Modeline, Driver and Modes with the correct entries for your system. Depending on your hardware, you may need additional entries, for example if your system has more than one graphic chip.

Section "Monitor"

    Identifier    "Monitor0"

    Modeline "1280x1024_60.00"  109.00  1280 1368 1496 1712  1024 1027 1034 1063 -hsync +vsync

    Modeline "1024x768_60.00"   63.50  1024 1072 1176 1328  768 771 775 798 -hsync +vsync

EndSection



Section "Screen"

    Identifier     "Screen0"

    Device         "Card0"

    Monitor        "Monitor0"

    SubSection "Display"

        Modes       "1280x1024_60.00" "1024x768_60.00"

    EndSubSection

EndSection



Section "Device"

    Identifier    "Card0"

    Driver        "nvidia"

EndSection

If you don't know the name of the video driver that your system is using then you may get the name as follows (if you have an intel graphic chip, the driver name is just "intel"):

lshw -class display | grep "driver"

The modelines can be generated with cvt:

cvt <h-resolution> <v-resolution> [refresh]