I would spawn things in in a sandbox save, then convert it to a normal save and use the mailbox
DO NOT convert your normal story mode save to sandbox because a few things get deleted permanently for some reason (like maxwell and some of the argemia plushies)
to change a save's gamemode go to votv's save directory (appdata>local>votv>saved>savegames) and change the prefixes of your saves to match the mode you want them in (a_ambient, s_story, b_sanbox, and I'm pretty sure i_ is endless and l_ is solar)