If you are referring to using/moving existing vanilla characters to a custom game then yes, you would need to use GDStash to flag your character as a “mod” character in GDStash. Like so…
That is only the first step tho. The second thing you would need to do is you would need to manually move your characters save folder from “main” to “user” folder, like so…
…and if the user folder does not exist yet you would create this folder and then move the newly flagged character folder into this location.
There is only one way available currently to run “Custom” characters in Crucible. You need to use GrimInternals.
After that you then need to create a shortcut of the GrimInternals executable and then modify the shortcuts properties, like so…
Then whenever you want to run the character in Crucible you would launch this modified shortcut.
So, yes, everything you want is possible to do but there is some initial setup work that needs to be done first. After that tho it is fairly automatic.