DataSource Controls :: Create Interface For Adding Users - Datatable Or Dataset

I need to create an interface for adding users to .Net Membership DB through an existing database. My Idea is to read each row from database from existing users table in a datatable and then iterate through each row and add it to new database. I cannot directly copy the rows from existing users table because the password stored in .Net Membership provider is in ir-reversible hash code and hashing is done through encryption algorithm present in front end code. My User count is huge (in millions).So what should i use to read the records, Dataset or datatable. Also tell what is the max number of rows it can hold.