The CACert wiki has full information on how to modify and update your own CACert file:
https://wiki.cacert.org/FAQ/ImportRootCert?action=show&redirect=ImportRootCert#Android_Phones

Prerequisites:

On Phone
- Root access
On Computer
- Java 1.6
- Android SDK

HOW TO INSTALL THIS FILE

To write the file to your Android /system partition you have to remount it with read-write permissions.

## remount /system read-write
$ adb shell mount -o remount,rw /system

## If this command does not work on your phone, use mount to find out how /system is mounted.
##  E.g. on a Samsung Galaxy S you find a rfs file system and the working syntax was
## mount -o rw,remount -t rfs /dev/block/stl9 /system

## upload the new certs file
$ adb push cacerts.bks /system/etc/security/

## remount /system read-only
$ adb shell mount -o remount,ro /system

## Samsung Galaxy: mount -o ro,remount -t rfs /dev/block/stl9 /system
Step 6: Reboot your phone