Can I create new Membership accounts manually with OptimizeMember?

Yes you certainly can!

In addition, with PayPal® Pro (and with Authorize.Net®) you're supplied with a Virtual Terminal online; so you can also accept transactions by mail, or by phone. After you've accepted a transaction (or not, does not matter), just log into your WordPress® Dashboard, and go to WordPress® -> Users -> Add New. Fill in the Customer's basic details, and send the Customer their Username/Password via email. After creating the Customer's account manually, you'll want to set the Customer's Paid® Subscr. ID. This way OptimizeMember can take over with programmatic management going forward into the future. You can even set a manual EOT Time (EOT = End Of Term) if you like, which is optional.
In your Dashboard, navigate to WordPress® -> Users, and click "Edit" next to the Customer you wish to edit. The value of Paid® Subscr. ID, can be a PayPal® Standard "Subscription ID", or a PayPal® Pro "Recurring Profile ID", or a PayPal® "Transaction ID"; depending on the type of sale. Your PayPal® Virtual Terminal will supply this information. If you're using Google® Checkout, use the TID/SID value in the sale Description; it always starts with opm- ). ClickBank® provides a Receipt #, and Authorize.Net® provides a Subscription ID. The general rule is... IF there's a Subscription ID, use that! If there's NOT, use the Transaction ID.