I was wondering if anyone knows how to thicken a premade structure with world edit. I have created a custom structure, so I can not make it thicker by recreating it with a larger radius. Ideally, I am looking for a command that takes a selection and thickens all blocks outwards by one.
No, there is not.
It's not a practical command because how can worldedit tell what blocks needs thickening? however if your structure is a square/rectangle then there are two ways you may be able to achieve this:
1) Paste the structure then select each side and copy it out by one block (repeat for each side/wall).
2) Paste the creation a total of 5 times, firstly one block north, then east, then south, then West, then finally paste in the middle, now you will have a double thickness to all outer edges of the structure.