You can't. The roll choices are based on weight sets that come shipped with HB. The only time it will roll need is if it's an upgrade based on the weights contained within. If it's not considered an upgrade, then it will roll greed/disenchant. Currently, there's no way to set the bot to ALWAYS roll need.
Also, unless you're running a full bot group, it's not really a smart idea to roll need on everything. You will get player reports against you for doing that. Just sayin'...
Edit: Forgot to mention that it's AutoEquip, located in HB's main settings tab, that rolls on items. DungeonBuddy doesn't roll on them.