You should create the dollars bank account and register an inter account transfer between the two with the amount in Naira and the amount in dollars. You basically buy dollars at an exchange rate fixed each time. This amount of dollar will never change (unless you buy or sell something from this account).
As said after that you will buy items in dollars directly from your dollar bank account.
A completely different thing is how to keep update the values of your assets and liabilities that you have in dollars inside your BS.
You shouldn’t input each day the exchange rate, even if you can and maybe it is necessary due to big exchange rate fluctuations between Naira and Dollars. But you should, at least, at each closing of your accounting period.
This exchange rate registration will determine, at each closing time, the updated value of your assets and liabilities, ie of your inventory in dollar, of your dollar bank account and of your liabilities for unpaid dollar invoices and so on.
By the way, this has very little to do with how Manager works. These are the basic principles of multi-currency accounting that I invite you to deepen and study.