Adding new fields to wordpress tables

Is it ok to add new fields to the existing wordpress tables without worrying that future wordpress upgrades will break the plugin?

For example, say I want to add a new field to the _posts table to add additional information for each post. Would this be ok to do? I would just create a new table but it would be a 1 to 1 relationship and that goes against normalization standards. I would also like to add new fields to the users table.