How to move sram to top hierarchy in Chisel3/Firrtl

Question

I make a RISCV core with chisel3. However, when I want to tapeout the core, the SRAMs make a big problem. I should synthesize the logic first, and synthesize the top hierarchy with SRAM as blackbox.

So how can I move all SRAMs to the Top? I have tried the topWiring. But I find it can only wire the output port or the wires to the top, instead of the module. Can anyone provide me with an example?