You have to edit the bibliographic data, see https://explore.researchgate.net/display/support/Reviewing%2C+featuring%2C+and+editing+your+research for instruction. There is a field for an existing DOI. - If your conference paper does not have a DOI, the situation is more complicated. For published works (including conference papers), ResearchGate will not assign DOIs, only for unpublished research (see https://explore.researchgate.net/display/support/ResearchGate+DOIs). You may follow the recommendations in the following discussions:
Borrowing from Rob Keller: The best and most easy way to create a DOI for a paper of a journal that don't assign DOI number or chapters of previously published books without a DOI etc. is Zenodo: https://zenodo.org/