I can see, there is no option to fill up exchange rates in lsmb directly. I know, it is possible to fill it directly to the database.
Why it would be nice to be able to manage exchange rates:
One want to issue an invoice in foreign currency. One will write a number (copy from somewhere else): 221.34 and insert to the FX field. Post the invoice and realize, he/she screwed the invoice, because it expected 221,34 instead of 221.34 and now one get 22134 as FX. This can be a common issue, if the number must manually entered.
One create a recurring invoice in FX. Next time one wants to create a new transaction by using recurring transaction menu, but not possible to create the invoice, because there is no FX yet in the DB.