The right for an author to self-archive a copy of a conference paper is determined by the terms of the publishing agreement.
Unfortunately there is not yet a comprehensive database of publisher policies summarising the self-archiving rights for conference papers. Sherpa Romeo may provide some information on conference proceedings with valid ISSNs. Researchers will generally need to refer to their publishing agreement for specific author-self archiving rights.
Published versions of conference papers are seldom permitted to be made openly available in espace unless the author retains copyright or the proceedings are made available under an open licence.
Conferences organised by large professional organisations such as IEEE often provide specific guidelines on the version of the conference paper suitable for self-archiving in an open access repository. Alternatively authors can contact the conference organisers directly for written permission to deposit the accepted version of the publication to espace.