How can I build in the outpost or any restricted build area as an admin?

Google "UberTool plugin" and you will find the answer to your question. Or just try Build plugin from here.

Build does the trick.

For example, If I wanted to place vending machines at the Outpost in a 1x1 for example. can this plugin in do it?

Yes.

However, if your intent with the 1x1 is to limit player access to the back end of the vending machine, this can spawn the machine without the doorframe, and spawn it anywhere. So you can spawn the Vending machine halfway through the wall or in a doorway in the outpost, so that it's back is only accessible by noclip, and then don't need to take up a 1x1 space in Outpost.

The alternative solution is to edit your map, use one of the Outpost replacement prefab available. Personally, I would recommend using RobJ's Custom Outpost, which has a variant Outpost with actual buildable areas in, and no safe zone or NPC's. You can still kind of enforce safe zone rules by placing sentry turrets around, and the players love the novelty of building inside the Outpost.

Can the vending machine auto restock?

Vending Manager plugin : /set Bottomless Immortal while looking at the machine.

thanks lots man