Store the player's team in a variable, clear said team, add the Rotom, and then, after the event has concluded, get the player's team from said variable using the "Script" command in the Event Commands for event pages.
$game_variables[55] =
$Trainer.party.clone
This stores the player's entire party in one variable, perfectly replicating their movesets, IVs, evs, natures, nicknames, balls, etc.
$Trainer.party.clear
This clears the player's party and purges the team. (Of course, you'd add Rotom to the team for this instance)
After that, use $Trainer.party = pbGet(55) to retrieve the team stored in Variable 55 at any time. (Note: After getting the team from Variable [X], the variable will be left completely empty. If you want to keep using variable [X] to swap teams for multiple events, be sure to copy the player's team before all else.)
Variable 55 can, of course, be any variable you want, I'm just giving an example. =w=