Applying dirichlet bc on derived quantity

Hello

I have a system of two equation with two unknowns, say "rho" and "m" = rho*u.

I am solving for rho and m but the BC is specified in terms of u. Is it possible to apply such a BC ?

In finite volume, I would update rho at boundary cell and set m = rho*u for the second variable (i.e., dont update m on the boundary).