That is very easily modded, all one needs to do, is to turn 'is outside furniture' to true in each item's file.

Step by step:

1) open FCS
2) create a new mod and give it an unique name
3) navigate to buildings and furniture
4) open the file (or all the files) you want to turn into outside furniture
5) change the 'is outside furniture' value and save
6) load your mod after all other mods.

you might give advanced camping a try as well:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2609558549&searchtext=camping