With numbers that high, feeding everybody would require artifact-level magic. Magically creating food is hard because there just aren't enough spellcasters to go around. Stockpiling lots of wands of create food might make a small dent, as each 50 charges could feed a total of 750 people for a day, but it would be more cost-effective to stockpile actual food.
One short term solution is to set up a permanent teleportation circle from someplace that's not under siege, so food can be easily brought in. Set up an outgoing one too, so the refugees can escape. These will help the immediate crisis, but will lose their usefulness as other portions of the world fall under siege.
Gate could let some refugees escape to a peaceful Outer Plane. Unfortunately each casting only opens the gateway for a short time, and you may annoy the current residents of the destination plane.
The only long term solution I can think of is to break the siege. That's what you're good at anyway. Adventurer powers are made for quests and violence, not for disaster relief or humanitarian aid. Give the local rulers what assistance is immediately available to you, then set out on a quest to destroy the enemy forces. (Look for the focal point-- kill their leader, steal their sacred artifact, close the portal to their home plane, whatever.)