Let users manage certain field I'd like to give a set of users the ability to manage certain fields, such as adding items to a list. Is this in any way possible with existing modules?

Example:
Editing a node
Try to select XYZ for field FOO
XYZ isn't an option
Go to admin page and add XYZ to FOO
Can now select XYZ from FOO
I think the module Field Permissions allows you to do what you're looking for. Or at least in the documentation it write "Create own value for the field".