Can I use my FSA and HSA with ICHRA?

Yep! If you select an HSA eligible plan, you can use your HSA to cover your out of pocket medical expenses and your ICHRA to cover your premiums. The same goes for if you have an FSA in place.

How do I add my FSA and HSA to my ICHRA benefit?

FSAs, HSAs, and ICHRAs are all independent funding mechanisms to help cover the cost of healthcare. As such, you can't add them to each other. Here's how I would think about it:

ICHRA - mechanism for employers to help employees cover premiums

FSA - employer owned flex spending account that employees can use to cover spot up medical expenses (regardless of the health plan they're on).

HSA - employee owned flex spending account that employees can use to cover spot up medical expenses (if they're on a high deductible health plan)