You can transfer you character to custom game buy copying his/her folder to
C:\Users\USERNAME\Documents\My Games\Grim Dawn\save\user
and then opening this copied character (not the original) in GDStash and toggling in mod flag.
All his personal items will be copied.
As for stash you can either move all items from it to your character before the whole operation
or you can
- select your Unmodded stash in GDStash and set it to your mod
(this will move your stash from
C:\Users\USERNAME\Documents\My Games\Grim Dawn\save\
to
C:\Users\USERNAME\Documents\My Games\Grim Dawn\save\YOUR_MOD_FOLDER
If there’s a message that stash already exist, you need to delete your current modded stash from
C:\Users\USERNAME\Documents\My Games\Grim Dawn\save\YOUR_MOD_FOLDER