You can make an actor using DECORATE (http://zdoom.org/wiki/DECORATE), load it on Doombuilder and place it anywhere you want.
For example, something like this if you need a new custom powerup or inventory item:
Actor LioHealthBonus : CustomInventory
{
+COUNTITEM
+INVENTORY.ALWAYSPICKUP
Inventory.Amount 1
Inventory.MaxAmount 200
Inventory.PickupMessage "You just found the hottest health bonus in the world."
inventory.pickupsound "misc/healthbonus"
States
{
Spawn:
BON1 ABCDCB 6
Loop
Pickup:
TNT1 A 0 A_GiveInventory("HealthBonus",1)
Stop
}
}
Or this if it's just some decoration:
Actor Barrel
{
Radius 16
Height 20
+SOLID
states
{
Spawn:
BARR ABCDCB 4
loop
}
}
Just don't try copying these, they won't work for you, specially the barrel, it's just random code I just made up. Just use them as a guide and use ZDoom wiki if you don't understand something or for reference.