if a player gets called up due to injury at NHL/AHL level, the ECHL team is allowed to sign a replacement player as long as he is not in someone else's system. Madore was undrafted by the NHL and his contract with the Express had ended so he was in a sense a free agent. I said after the last game of the year that he put on one of the best performances for a goalie I had seen at any level in the third period and obviously he stayed hot.
Teams can only add players to their rosters in emergency situations which means the number of skaters drops below 16 or below 2 for goalies.
When SC's starting goalie called up it dropped the 'Rays number of goalies below 2. Since Madore was under contract with Chicago he technically became a free agent as soon as the Express ceased operations. Other non-affiliated players from other ECHL teams not in the playoffs, Dan Eves for example, could not be signed because he is still under contract with the Cyclones.
Here's a link to the Playoff roster guidelines for future reference: