Typically, this request comes from a less-than-reputable conference that collects money to get papers published in journals. The Board of Governors has a policy on this issue. The author (and not the conference chair) needs to submit the paper to your journal, noting in the paper itself that an earlier version of the paper was published in the conference’s proceeding and citing that paper, and it must have 30% or more change of a substantial nature; that is, not simply padding with more words. You then have the paper reviewed as normal.