Can I add an "author contributions" section to make clear that I did all the work on a paper?
I think you need to consider for a moment if this is in your own benefit.

Depending on how you go about it, adding such a section without your contributors knowing can come off as a backstab, and hurt your relationship with your contributors. Especially since the paper is already under review, and you seem quite adamant to have it included. If you do let them know (and review) beforehand, it might still come off to them as off-putting that you are adamant about discrediting their contribution (in your own words "I did all the work"). Sure they might not have written the paper, but is it really true that they contributed nothing of note? It is very common for supervisors who do reviewing or who only give advice to be listed, even if they have not written any original text. Other academics are aware of this, so what do you really gain by asserting yourself as the sole author? 

If you do chose to include a contribution section, make sure that you go about it in a way that does not hurt your relationship with your colleagues, something which is far more important than the exact credits of one publication.