The domain of a standard data element cannot be changed without a modification. It is also not recommended in general. If the field is a custom one then you can use a custom domain. It is possible to add custom fields to standard transactions via designated user exits, so standard transaction <> all standard fields.
You might want to ask for a mentor to be assigned to you while you are learning. Also - just look at how the same things are already done in SAP. E.g. your first question could have been easily answered without SCN if you've just looked at any configuration table (T*) that has a text.