Can you expand a bit on your question?
When you say users from a custom form, what specifically do you mean?
If you mean that the user would be provided a field in which they could type any name in the world - I don't think that would work - I do not think Proof HQ will have access to data in custom forms.
I am not aware of a custom form field which will provide a user with a list of system users.
I think the general idea with proofing workflows is that you have several created for different scenarios and when the proof is created you apply it. Maybe you end up with slightly different versions of the same thing. You always have the ability to add or remove people from the workflow once you apply the template.