Found the solution myself.

Odoo V8:

1. Go to 'about Odoo'
2. Activate the developer mode.
3. Go to your view you want to edit, for example to sales > products and open up a product.
4. Open the debug view (top left) and click on edit FormView
5. Find the field 'inherited view' and open it.
6. Edit the code under the architecture tab in the new window.
6A. You should first add the fields you want to add to your view in your database. The model where you should insert this is in the 'Object' field, for example product.template. You can add fields to a model by going to settings > Database structure > models. Find the correct model there, make your new fields, give them the correct type and save them.
6B. Edit the code under the architecture tab and add the fields with the same name as you've named them in the database.
7. Save and refresh your page, you will now see your now tab with fields.

I've also made a guide about this in image form with even more information which can be found here: http://imgur.com/a/1C1gK
 

Odoo V7:
1. Go to 'about Odoo'
2. Activate the developer mode.
3. Go to your view you want to edit, for example to sales > products and open up a product.
4. Open the debug view (top left) and click on edit FormView
5. Edit the code under the architecture tab.
5A. You should first add the fields you want to add to your view in your database. The model where you should insert this is in the 'Object' field, for example product.product. You can add fields to a model by going to settings > Database structure > models. Find the correct model there, make your new fields, give them the correct type and save them.
5B. Edit the code under the architecture tab and add the fields with the same name as you've named them in the database.
6. Save and refresh your page, you will now see your now tab with fields.

I've also made a guide about this in image form with even more information which can be found here: http://imgur.com/a/UphvK