[member="Lisette Kuhn"], [member="Alva Calvarona"],
There are no rules regulating Force Users against writing themselves as members of the Imperial Navy, Imperial Army, Council of Moffs, Imperial Intelligence, or restricting them only to the Imperial Knights. Any writer can join any sub-faction, with the exclusion of NFU joining the Imperial Knights. What it excluded, is a PC picking up two hats and being official members of more than one sub-faction.
Moreover, the "Member World" theme exists to help cover the gaps. Much like the relationship between the Witches of Dathomir and the Mandalorians, Member Worlds within the Galactic Empire are free to operate as they wish and free to develop whatever Force Oder they desire. So long as they are not directly undermining the Empire or building a darkside Force Nexus on their planet, the Galactic Empire leaves them alone. The Galactic Empire also welcomes those within the Member World's organization who wish to leave their world and interact with/aid the main faction. "Fleets" and "Armies" under the control of said individuals can fight alongside Imperial Navy fleets and Imperial Army battalions controlled by others. For those who wish to have a more diverse background and be a member of a Member World's minor faction, yet still command the iconic Imperial Navy or Imperial Army, then the capacity to do as much still exists. A Force User could choose to be a member of the Imperial Army or Imperial Navy, yet still be a member of a Force Order that is specific to an individual Member World.
Just keep in mind that (IC) it is the duty of the Imperial Knights to keep an eye out for those who could pose a threat to the Empire. Should a character that is also a Force User begin to behave strangely, the writer of that character should welcome the roleplay opportunity of being investigated and having an Imperial Knight try to work with that character to personally bring them back to the light.