Can we add articles downloaded from Sci-Hub to a meta-analysis study?
If you disclose that you downloaded it from Sci-Hub, one cannot rule out that you will suffer legal consequences one day (alongside, perhaps, social punishments based on moral grounds). But given the almost non-existence of a legal enforcement against the individual use of this 'pirate', I find it unlikely.
To be on the safe side, you should simply not disclose the source from which you downloaded that publication. Mention it, discuss it, cite it, but do so without telling the reader how you acquired the PDF. This would be too much detail anyway.
(Yes, a meta-analysis would do good to disclose how it came to include certain studies, but it does not have to tell the reader in minute detail how you downloaded each study's PDF.)