Simulate transient heating of a slab in a furnace
 

My master's thesis is a numerical simulation of walking beam type slab re-heating furnace, in which a metal slabs passes through the furnace to be heated.
I built the furnace geometry, with the 29 slabs in its interior, defining the furnace as a fluid zone and the slabs as a solid zone.
I want to make a primary run, simulating the furnace without the slabs, with the purpose of imitate the starting of furnace. Only when convergence is achieved in this first simulation, the different slabs are inputted in their positions. From here I will do a secondary transient run, for a period of time equal to the residence time.
My problem is, if the furnace already has the slabs in its interior ( choice that I made when I'm building the furnace geometry) I don´t know how I can removed them for the first simulation and input them latter
My idea is to identify which cells in the mesh belongs to the slabs ( which cells are solid ) and then change them to fluid. In other words I have two cell zones ( one solid, the other fluid ) and I want to transform the cell zone solid into cell zone fluid for the first run.
Is this possible? Can I do this with a user-defined function? Is there other way to do it?
On other hand if I start with an empty furnace ( choice that I made when I'm building the furnace geometry) and do the first simulation how can I place the slabs inside the furnace in this converged first solution, with the purpose of running a transient simulation?
Is this possible? Can I do this with a user-defined function? Is there other way to do it?

Appreciate all the help that you can offer