So if you want to use a specific backup account, put the Backup Account into the Administrator group on the EV server.
Then go into Roles Base Admin and give the account appropriate access - Power User (I think it's called that) is good - because this role has the required access to put both Index and Storage into Backup mode.
Though as scanner said, use the EV service account to start the DPM services as you will know it has the required privileges to do the job.