What a wonderful question! It's never too late to add sources!
Click the Edit tab on the profile. Scroll down to the section where the biography is. Add away. :D
If you want to add footnote sources,
- go to where you want the footnote reference to be
- type <ref>
- type the text of the footnote
- type </ref>
WikiTree will do the number & put the footnote wherever <references /> is - provided <references /> is AFTER <ref>text</ref>