In stellaris your faction mechanics are determined by your ethos, never being able to colonize a planet and live on huge arc ships would somehow have to be shoehorned into that. And before someone says something about the nomadic trait, no that does not determine your playstyle or tech pool. Traits are often picked to complement one's faction, but are not strictly tied to the playstyle or factions mechanics. After all, having very strong citizens does require you to invade enemy planets, nor does having intelligent population give you access to exclusive techs on the research deck, etc... Having the nomadic trait alter the way a faction plays would be breaking continuity with the other vanilla traits. So being nomadic would have to be shoehorned in on the ethos branch. So do we have a 5 way ethos branch now? And this post is getting too long winded for me to dive into the ramifications about that.
This suggestion shows one of Stellaris's downsides. The faction system is extremely flexible and customizable, but it's wide and modular reach means it cannot go very deep. While In the endless franchise there is no such restriction and each faction's gameplay is custom tailored to their lore and meant to be totally unique from the get go. And in the case of the roving clans (Endless legend nomads) and the Vodyani, their games respective mechanics were designed from the ground up with them in mind since they were debuting factions. Like the sheriff said, a Stellaris nomad would have no such support.
So could they shoehorn in nomadic gameplay? Sure. Will they be able to pull it off effectively in the constraints of Stellaris game mechanics? I would not have high hopes for it.
IMO Ethics wouldn't determine if you are Nomads, it would determine what you do as Nomads.
Militarist nomads would probably get what they need hiring themselves as mercenaries, or raiding lightly protected worlds before the empire that owns them can respond. Pacifists could be traders, acquiring resources from one empire and trading them away to another. A xenophobic Empire might avoid direct contact with empires diplomatically, gaining what they can by raiding and/or illicit smuggling, while Xenophilic Nomads would regularly interact with earthbound empires, allowing those that wanted to join them to do so. Spiritualists are on the fleet as a pilgrimage. Materialist for the chance to boldly go where no blorg has gone before (and make some energy credits from it), and Authoritarian/Egalitarian is based on whether everyone in the fleet is expected to dedicate themselves entirely to the fleet, or everyone is here because living a life traveling the stars is seen as the ultimate expression of freedom. And if you're Hive Mind, Petheryon Swarm says hi.
If you make it a choice, it's probably either a starting civic (like Syncretic Evolution) or a new overarching option related to homeworld or goverment type, with some new civics of it's own.
Some of the things here would require new mechanics, that is true, but there is nothing with the current ethics wheel that makes it impossible to be one.