I found a profile that almost matches my needs. Can I add some fields?

The fields of a profile are fixed, so you will need to use a different profile. Don't worry, you can create your own. Since you found a profile that seems to almost match your needs, the most logical thing to do is to create a new profile based on that one.

You can do this yourself, as long as you have a way to login to the Component Registry. Click the 'login' link and select your home institute or another provider you have an account with. If none is in the list, create an account with CLARIN (more info).

When logged in, select the base profile and click the 'Edit as new' button. Save it in your private workspace (under a different name and/or group). The profile consists of links to a number of components (some of which in turn consist partially of links to components), so you will have to identify the components that you need to change. Edit these 'as new', as well and make the required changes. You may have to do this recursively for deeper hierarchies. Then, in your profile, replace the references to the original components with references to your new versions of these components. Save the profile, and test it in an editor  (e.g. oXygen or Arbil) before publishing (you can get the XSD link by selecting the profile in the component browser and choosing 'Show Info' from the drop down menu on the far right. You can open this link in an XML editor or validator; in Arbil you can add it via the 'Profiles and templates' settings.