I would first make sure you're absolutely attacking the workouts as written before you add volume. You run the risk of (consciously or subconsciously) 'saving something' for your interval training later in the day. This can be especially true if training alone without a partner.
If the program specifies running without a pack and your test is WITH a pack, instead of supplemental interval training I'd add a weighted pack first because that is your task.
If you insist on supplemental interval training, try and keep your ACTUAL work time to 10 mins or less. (400m takes about 60-70 seconds, so 10x400m = 700 seconds or 11 mins and change. Which is too long. So 4x400 with 1-3 mins of rest between intervals, then another block of 4x400m after 4-6 mins of rest would be better.