Inserting a "User Profile Field" ?

[tpearl5]

I'd like to create a plugin that inserts a user profile field, in this case one that is a yes/no selection and appears under the user options. I realize I have to run a query to insert the field into the user table, but then how do you go about registering the field as a variable and using it in showthread/postbit hooks?

If someone could point me in the right direction I'd highly appreciate it.