Hi Sarah,
The answer is yes, you can install DirSync for EOP and in fact it is required.
In an EOP standalone environment, the user created in EOP directly (method that is creating users via CSV) cannot be used with EOP functions. In other words, in an EOP standalone environment, DirSync is necessary and the users’ attributes (for instance, the proxy addresses/aliases) need to be managed on the local AD side.
Moreover, our forum focuses on the questions about EOP that is built-in in Office 365 tenants mainly currently. If you need further assistance about EOP standalone version, we have another support channel that is dedicated to EOP standalone tenants in the following website:
http://technet.microsoft.com/en-us/library/jj723132(v=exchg.150).aspx
You can refer to the Assisted support from Microsoft section of the article above to contact EOP support directly.
Thanks,
Bourne Zhang