If the godparents are biologically related then they can be put into the family tree at the correct location - such as uncles, aunts, etc. A link to their profiles in the childs Bio Text box is also acceptable.
If the godparents are not biologically related then they can only be mentioned in the Bio text box - with a link to their profile.They cannot be attached as family members because they are not biological family members.
If you dont have any family details for a non-related godparent, then you choose to not create a profile for them, and just mention them by name on the childs profile.