From Worms Knowledge Base
This one is called in just like an air strike, only instead of bombs on the targetted area, the plane drops an extra worm, which drifts down by parachute to the ground. The worm then becomes a part of the team that called him in. From that point there are different options: either the worm who called in the reinforcement could use his retreat time as normal, or the retreat time could be given to the reinforcement worm, so that he may gain some cover when he lands. Alternatively, the turn could not end at all and the reinforcement worm takes the rest of the turn time to do whatever it wants.
For avoiding having an unlimited number of worms, this tool can only be used if there are less than 8 worms in the team. Also, after using it, you can't use it again until 2 of the worms on your team die. When the worm it died would it not leave a grave so it couldn´t be used to make infinite worms in a game using it in combination with the Resurrector.
Another problem is what name to give this worm. It could be simply named Reinforcement Worm 1, it may be given a random name, or it could be given the name of a team member who did not make an appearance in that round. The health would be defined by the starting health as denoted in the game options.
Programming the addition of an actual worm into the game and then making him apart of a particular team could end up with many problems, however.
Strategically, though, if things are going badly and you need help then the reinforcement worm could just turn the tables on your opponent. Especially if your turn doesn't end and you get to use your new worm to do something creative.
Reinforcement worms seem unnecessary when health crates do the similar job of giving you a quick hand, but then again having an extra worm may leave you with more options towards victory.