My suggestion is this: a free form field for properties, and an "Unknown" item in each category where the user can change the name. Give it a question mark icon perhaps.
As it stands, there is really no reason other than convenience why item properties (Dmg, CrtH, DrainX, etc) have to be selectable from a drop-down. In fact, there are so many properties that I find the drop-down selection process is actually more tedious. I would suggest that all item properties be changed to a free-form format where you can just type in the property and then select the multiplier. At the very least, perhaps it can be a drop-down that also has text entry/edit capability.
Secondly, if you put in an Unknown item in each category where the name is editable, then anyone can add items to their build which are currently missing in the planner. As a bonus, periodically the maintainer can search for all the item names that people have used the unknown entry for and then you will have a list of items that are missing that need to be added. This will mean people don't need to make a forum post for each missing trait, weapon, item, or property.