I have had a few clients who have been using DropBox to store all their files.  They have gigabytes stored there.  Have you considered a migration tool to bring those files and folders from DropBox to Zoho Docs?  

An alternative could be to integrate with DropBox like you have done with Google Docs although this would mean clients would be split to between two providers. Not ideal from a reseller point of view.

Regards

Richard Price

www.digital360.co.nz

A Zoho Alliance Partner