can cmiCopyDocumentFromAppian be called from a script task?...

Yes, you should be able to. One thing to note is that if you want to use the local-credential, you need to activity chain the script task to a user input task. In this case, the document will be uploaded as the person who completed the task (or if you activity chain to the start event, it will be the person who initiated the process)
The other option is to run it with site-wide credential.

Currently I am using a site wide credential.

Can we call rthe function directly? We would have to use a writer. Please do guide me on how we would go abbout it.

a!cmiCopyDocumentFromAppian(
scsExternalSystemKey: cons!CMIS_SCS_KEY,
usePerUserCredentials: false,
atomPubUrl: cons!CMIS_URL,
repositoryId: cons!CMIS_REPO_ID,
objectId: rule!CMIS_cmiGetObjectIdByPath(cons!CMIS_ISP_SSCM_ROOT & "/" & ri!folderName),
properties: {name: "Arch.jpg"},
appianDocument: ri!appianDocument
)

Hi Karls/ Sathya,

Is this possible in a script task? I read that that cmiCopyDocumentFromAppian requires the bind function. My understanding, the bind can only happen in the sail.

Your understanding is correct. Any writes should happen as a part of SAIL bind fn