Or HaChaim on Numbers 31:18/אור החיים על במדבר ל״א:י״ח

1 החיו לכם, "keep alive for yourselves." The meaning is that those who were allowed to live should convert to Judaism and only when they were Jewish would they qualify for the description חיים, "to be alive," as then they would be allowed to get married and to remain "alive" by means of the children they would bear.