your question is not clear to me, what I understand is , you already have some assets like image, docs an all and you wana use it in aem.
If I am right then my answer will be yes You can do it.
But approach will be different in different scenario.
If number of assets are less then you can manually upload it in aem's dam from your local machine.
If number of assets are huge, you can write some service that will get the assets from the already available repo. and write it into the aem dam with required rendition with the help of Assetmanager API.
We already have Sceene7 and s3 available that can also help you in some scenario.