That’s correct. I raised a request for this a few months ago – essentially a “Make Into Systems” function for Dorico, to go along with Make Into System and Make Into Frame.
Very broadly, select something in the desired first measure and something in the desired last measure, and invoke the function. The first measure gets a system break if needed (so that it’s at the beginning of a system), and then Dorico distributes the measures over the desired number of systems, keeping the system fullness as consistent as possible (i.e., preferring 75/75/75 to 100/100/25).
Edit: See Reflow selected systems