You can add/modify A records to your domain in the Dreamhost control panel, under Domains->Manage Domains, then select the DNS link under the domain to which you wish to add the records.
Please note that if your domain is set to fully hosted and you add a blank or www A record, you will experience intermittent success as your domain is now juggling between two different A records. You will need to set your domain to "Host DNS only!" if you want just the custom A record to work.
Sadly, if you are looking to transfer your hosting elsewhere we have some helpful wiki's below. If you have any other questions feel free to hit us up