I'm having this same issue.... I don't want to use the "Administrator" Role to manage accounts.

I never did find a good answer to this question. But through a lot of poking around, I found a way to do this. Though it might be a little shunned upon since it requires editing the core module code, so use with caution:

The easy part (which you appear to have already done) is to get the user to the form, either create a link/button somewhere to either User Profile or the Manage Users interface. You also might need to grant access for the custom role to Admin -> User Accounts in Page Management. 

The tricky part is to go into DesktopModules/Admin/Security/ManageUsers.ascx.cs   and edit lines 383-388 to allow the custom role in addition to the Admin role. I wrapped the current check with my own custom check:

//Check if user has admin rights 
DotNetNuke.Security.Roles.RoleController cRole = new DotNetNuke.Security.Roles.RoleController();
DotNetNuke.Security.Roles.RoleInfo iRole = cRole.GetRoleByName(0, "Admin - Users");
if (!UserInfo.IsInRole(iRole.RoleName)) 
{ 
... {original code which can throw NotAuthorized error} ...
}