How can I add a user field to sigma, for the CRM?

How can I add a user field to sigma, for the CRM?

In Zoho CRM it is possible to change the owner of a sales order, however outside of the CRM it's not possible at the moment. 
To work around this we have been trying to put together a sales person form in the CRM that will allow for an intermediary table. However, as this will be used by a creator app I am trying to add this form to our sigma extension rather than creating it in CRM as an ad hoc form. This way, once they have both been released on their respective market places, I know that the form will be there.
However, it doesn't seem to be possible to add a user dropdown to a form in Sigma.

How can I do this, and if I can't are there any plans to allow it to be done?
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