Yes, almost all JJ users do this constantly. Just "track" the particular branch. JJ has an idea that only some commits are immutable, the set of "immutable heads", and the default logic is something like "The main branch is always immutable, remote branches are immutable, 'tracked' remote branches are mutable." In other words, tracking a remote branch removes it from the set of immutable heads.

So just run:

    jj bookmark track myname/somecoolfeature --remote origin

and the default settings will Do What You Want. This is intended as a kind of safeguard so that you do not accidentally update someone else's work.

Some people configure the set of immutable heads to be the empty set so they can go wild.