If you own the dedicated server the account is on, make the user a reseller using the reseller center feature in root WHM.
If you are a reseller yourself, you won't be able to offer subresellers unless your host uses RVSKIN or WHMreseller and permits you to have sub-reseller accounts and even then your sub resellers won't have direct WHM access, just access to the particular features they need to be able to resell accounts.
