We had to do this when we first installed WHMCS. The procedure we used was a little lengthy but was something to the effect of:
First, add a new client in WHMCS ( Admin website --> Clients | Add New Client)
Next, Go to Orders | Place New Order.
Select your new client and check both "Don't Send Order Confirmation Email" and "Don't Generate Invoice", click Continue, and create the order.
The order should be created and put into a pending state (depend on your config). Go to the pending order and uncheck the options (if present):
Send to Registrar
Send Confirmation Email
Create Account
Send Welcome Email
If it is a hosting account, set the server, username, and password (if known). Then click Accept Order and you should get a "The order has now been successfully activated" message.
Then we went into Clients, selected the client and went to Products/Services. There we adjusted the payment and billing cycle info to match up with our client's next due date. Make sure to "Save Changes". Adjustments were also made to the domain dates, payment, and billing cycle.
It took a while but we initially only had 15 clients to move. Someone else may have a better/automated way.