Under Super Privs:
WHM » Resellers » Edit Reseller Nameservers and Privileges > Reseller, Super Privs >
Account Modification (warning: this will allow circumvention of account creation limits, give shell access unless explicitly disallowed, dedicated ips, etc)
The reseller will still only have access to accounts that are owned by the reseller username. You can change ownership of accounts to this reseller via WHM as "root":
"WHM Home » Resellers » Change Ownership of Multiple Accounts"