I’m not sure if I totally understand the question… But it sounds like you want to be able to send an invoice for, let’s say $1, without having to do the conversion manually? I can do this in Wallet of Satoshi… See 2 photos.

First click the lightning thingy, then add the dollar amount. That’ll bring you to a new screen where you can enter the amount and if you notice a little up-and-down arrows that’ll toggle between fiat and Satoshi’s.