I have a Sapphire Pulse RX 6400 card floating around, a 5700G APU, and a Asus Tuf Gaming B550 board. I know that the board supports crossfire, the question though comes down to the card and apu.
AMD ended Crossfire a few years ago so it is considered to be legacy and not supported anymore. AMD MGPU (Multi-GPU) technology took the place of Crossfire: https://www.amd.com/en/support/kb/faq/dh3-018
To ensure system compatibility with AMD MGPU, please follow these guidelines:
Here are the requirements for Crossfire from AMD Support: https://www.amd.com/en/support/kb/faq/dh-018
These are the requirements to enable AMD CrossFire™:
Once all AMD CrossFire™ requirements are met, the graphics hardware and software must be properly installed before configuring AMD CrossFire™
For instructions on installing your GPU hardware, please refer to the product user manual or documentation. For instructions on installing AMD graphics drivers, please refer to KB: How to Install Radeon™ Software on a Windows® Based System.
So from the above requirement for both Crossfire and MGPU doesn't seem like you can pair an IGPU with a GPU card.