You could create a requester group called managers and have the criteria be anyone that has manager in thier name and then add that requester group into the ‘employee’ stakeholder Module that way HR and Managers can request an onboarding. then if you don’t want HR to be the next step in “approving” you can make an automator with a conditional to avoid that step or use an api to auto approve it.

I’ve not used the onboarding module yet but is there a way to create two different onboarding workflows so one is Employees and the other is Contractors?

In our case we had the same situation as you but instead we changed business practices so that HR is involved with all hires employees and contractors (interns, temp workers, etc) this way HR ensures that SOWs, NDAs, required security training, Policy acknowledgements, etc are signed by contractors.  In our business managers rarely bring in additional talent and would forget the process so standardizing to HR made sense for us.

The onboarding module only makes sense if you want to track the process: HR starts the process and then the Reporting Manager adds info and selects the kit.
If for contractors this process is missing I suggest leaving the onboarding form for hiring employees and creating a Service Request “Hire Contractor” in the Service Catalog visible only to Reporting Managers. This way you have full flexibility on the two flows. With the child items the Reporting Manager would be able to define the contractor's kit at the request stage