If my players place modified spawner, they will change to pig spawner. After adding essentials.* this didn't happen any longer. Could you tell me the exact permission node?

Got it, essentials.spawnerconvert.mobtype.
Maybe you could make a list with essx perms.

This is a standard permission node that was already there when the original Essentials implemented it, so you could just check their command reference? http://wiki.ess3.net/wiki/Command_Reference

Just realize that they didn't put it in the list.