Yeh, scripting system is not necesary for it.
Simply set up a common event that randomizes a number between 1 and x; and make a conditional branch within the brackets that performs actions when number = 1.
Inside the conditional branch bracket, make an event that changes party members and choose add ___, perhaps some 'summoning' messages before that.
Make an item that activates the common event. (And set the common event's trigger to none).
*Make sure there's a de-summoning item or something to get rid of it.
^Not very detailed but it's basically how you can do it.