Please note that you wrote to the ResearchGate community, not to the RG team. However, they do not add journals manually. See https://help.researchgate.net/hc/en-us/articles/14293139566353-Journals for explanations. But see also https://help.researchgate.net/hc/en-us/articles/24012471222545-Journals-on-ResearchGate for a possible cooperation between the journal's publisher and ResearchGate. I guess the publisher has to pay for this.