You can have any level service and add practically anything else that you need. That will cost you the price of the extra service from the date when you order it - regardless what other services you get.
You could also - in the middle of a "contract year" - upgrade or downgrade to another plan. If you do, then you simply pay (or get refunded) the amount for the remaining time of the year. Simple!
Perhaps you should check if the extra services are included in a bigger plan and upgrade to that instead of getting individual extras. It might save you money.
To upgrade the plan, go to Billing - Change Service: https://panel.dreamhost.com/index.cgi?tree=billing.service&
To add stuff, go to Manage Domains: https://panel.dreamhost.com/index.cgi?tree=domain.manage&
Try out DreamHost with a free WebID -- Prices, options