Hello, If you would still like to add meta keywords, I found this great script: http://www.prestashop.com/forums/topic/185401-free-script-product-mass-edit-category-multi-edit-order-edit/
With it, there is an option to edit and add meta keywords. When you 'view source' you will see your keywords have been added
I am happy with this, because I feel I would rather (and could be better) to have keywords added then nothing at all as it is still unsure whether keywords are useful and/or necessary for other search engines. I still cannot wrap my head around not adding keywords and will be doing some research as to what is best option and how your products/pages can be found without keywords? Again, I am very happy I have found way to add keywords if is your preference.