I have done this.  It was like pulling teeth.  I will tell you how I did it with the caveate that there is probably an easier way to do it, but I wasn't able to figure it out.

I started by hacking the portal security code.  I modified the portal security code that checks to see if a user has permissions.  The function that typically gets called for admin pages is IsAdmin().  In pseudo coe, Every where that was called, I replaced it with IsAdmin Or (IsController and Page name contains "User").  Additionally, there was a ascx file for the Add users page that I needed to modify to allow not only users of the Administrator role, but users of the controller role as well. 

All in all, it was probably 10 lines of code (if I recall correctly) to get it to do it... but it represented about a week of work just figuring out what to do in the first place. 

Here is the real problem,  if I upgrade my DNN installaiton, I will loose this feature. No worries, prepared to live with that for now.

Do you have a development environment to compile the core source?