Signals & Track How To Replace A Zs3/zs3v/zs6 With The Zs3f/zs3vf/zs6f Variant On Signals?

Hi,
I was wondering how to replace a Zs3/Zs3v/Zs6 light indicator with the Metal Sign version? I have a KS-Signal that needs multiple Signs in place of the lighting indicators. Unfortunately I can't select the Metal Sign version when editing the Signal on the bottom right under the "Heads" category. I found out that the Metal signs are only under "Signs" but I am wondering how to get the sign then in the correct position where the Zs3/Zs3v/Zs6 usually spawn.
Thanks for helping out