A basic DS3 save editor built specifically for moving characters between save files and transferring other players' saves to your account.
Please see the included readme for detailed instructions.
SaveMerge can also be found on NexusMods.
Requires .NET 4.8 - Windows 10 users should already have this.

To the best of my knowledge, moving characters and altering Steam IDs is perfectly safe on its own.
However, many mules and other saves uploaded on Nexus have invalid data such as inconsistent soul memory, impossible key items, etc, which will get you banned regardless.
Use this tool at your own risk.

Atvaark - save research
Meowmaritus - app icon

SoulsFormats by Me