Is it possible to prolong the barlines of a piano staff so that I can link this staff to another piano staff under. I also would like to move certain staves closer to each others and move other away. I usually compose on a 4 staves system. With Sibelius it is possible.

Second question : The XML export toward Cubase even though working very well doesn’t export certain dynamics. Only the F or p are exported, never the mF or mp.

You can get a 4-stave piano in Dorico by creating one in another program and importing that (empty) file into Dorico using MusicXML. Then add the other instruments you need.

Eventually I expect there will be an “instrument editor” in Dorico, but I don’t know when that will get implemented!