Alright, I found it.
It seems that you can specify a configured remote to the add
command via remote://your-remote/your-file
(instead of s3://your-bucket/your-file
)
So in my case I configured a second remote s3remote
for referencing files there:
$ dvc remote add s3remote s3://my-bucket
$ dvc remote modify s3remote profile MyProfile
$ dvc add --external remote://s3remote/remote-data.txt