You can list an incoming role if it is confirmed and relevant, but label it clearly as incoming with the expected start date. Do not write bullets that imply you have already done work there.

The question is whether it improves the recruiter's understanding of your trajectory. If the incoming role is materially stronger than an older experience, it can be useful context.

Keep the entry compact. Your completed experience should still carry most of the evidence in the resume. If useful, this guide on consulting resume guide gives more detail.

I'd list it as incoming if offer is signed. just don't invent bullets for work you haven't done.