To Use a Unused interior(The Interiors which won't able to access in the game.So they don't have a yellow
marker)for your mission follow my steps.
Step 1:Add a checkpoint objective to the place which you want to make enter-able for your mission.
Step 3:Browse the suitable interior for your mission and select it.
Step 4:Inside the interior Go to Objectives>Add objective>Special player Env. objective>Teleport. Now select model of your player.
Step 5:add the objectives you want in the interior.
Step 6:Now for exit add a checkpoint objective at the exit of the interior(unused interiors haven't any exit marker).
Step 7:At outside teleport you player model.
That's all!
Special notes
You can use the same method for changing the clothes/model of your player.Instead off checkpoint add a
Cloth pickup objective in that case.
*Checkout my missions please and comment if this helps you*