If you’re asking how to use ART on a character with a pre-existing skeleton then ART doesn’t support this.
One way is to save the file as a backup, remove the skin cluster and delete the joints, reference in the backup (ctrl+R), then do the setup with ART as usual and just snap the pivots of each control to the referenced pre-existing skeleton. After the process is completed, use Copy Skin Weights to transfer the weights from the referenced mesh to the new one.
This will work 100% if both skeletons are identical and I’d assume that the infiltrator demo would use default UE4 skeleton template.