We have no plans for a Dorico 4.5 release (Steinberg will not be releasing .5 updates for any of its products any longer), so the next major release of Dorico, when it eventually comes, will be Dorico 5.
You can certainly import Cubase expression maps into Dorico, but they’re not going to work exactly as they do in Cubase out of the box: you will need to review the switches and the actions defined in them carefully. However, they will provide you with a useful starting point, if nothing else.