I haven’t sewn the Marais dress (yet) so take the following with a grain of salt, but here’s what I’d do if I wanted to add a sleeve:
Choose the knit pattern that you like the sleeve/armscye of; eg metro T, or the Easton, or the Belgravia dress, etc
Trace both patterns in your size and find the centre front line (it’s the fold line for the metro and Easton, but a bit trickier for the Belgravia).
Overlay the bodice pieces of the pattern with sleeves over the pattern without (Marais) and then make a third tracing that blends from one pattern to the other.
Then you can just do some Building Block style fiddles to turn the sleeve into what you want.
Does that make sense?
It’s much easier than trying to adjust an armscye that was not designed for a sleeve.