Add a custom field to the newsletter subscription web part

What kind of information will this added field contain? Does it have to be in the subscriber table as well, or can it exist somewhere else?

We are looking to track the company name of the user signing up for the newsletter. During registration, we need to collect, first name, last name, email and company name.

Whichever way is easier and is the kentico way of doing things. Add the new column in the subscriber table or handled somewhere else.

The subscriber table is in Modules > Email Marketing > Classes > Newsletter - Subscriber

You also have to think about adding that field to the current webpart(cloning it and making your changes) and also the view in the back end. Right now, it won't support/show your added field, so you will need to clone that admin page in the subscriber module, similar to where you added the field, except instead of clicking the classes, you will click the UI tab and change the url of the module page, to point to your cloned page, with your added field in the grid.

This is for K7, but might be helpful

I guess we may run into issues during kentico upgrade, correct?

Not if you clone the controls and customize your cloned versions. Usually the database updates stay there and don't cause issues, at least from my experiences.