How to manage Job Offer after Move To HR

Hi Team!
I'm writing you to receive support in order to manage as well as possible a client request.
The client wants to manage the Job Offer after the preboarding is triggered, I mean after the Move to HR, cause of the candidate receives the Job Offer after passing a series of examinations.
The examinations are handled by using tasks in preboarding steps, so they want to create a Job Offer and, after acceptance, to continue the preboarding, so other tasks.

the preboarding tasks are triggered out as checklist from HCM.
In the middle of the process, before "generating contract" step in a third part system, the client would like to ask a "availability request" in order to effectively proceed ahead.