for Re = 100, the backfacing step has a steady solution.

In most literature, I see people solve such a problem using a transient solver and reach the steady solution eventually.

but, can I use a steady solver to get the solution with the first guess for iteration to be something like, for example, all zeros?

Certainly you can. Use All zeros is plausible. The next iteration, BC condition will drive the solution.

Kenn what made you think that solution of this problem with steady state solver is not possible (even with all zero velocities to start with ?)