I have done exactly this.  Theres a lot of code but youll see what I did

        Protected Sub cmdUpdate_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles cmdUpdate.Click
            If AddUser Then
                If IsValid Then
                    Dim createStatus As UserCreateStatus = CreateUser()
                    AddUserToRoles(createStatus)
                    UserCreateCompleted(createStatus)
                End If
            Else
                If ((UserEditor.IsValid) Or (ProfileProperties.IsValid)) AndAlso (Not User Is Nothing) Then
                    User.Membership.UpdatePassword = False
                    UserController.UpdateUser(PortalId, User)
                End If
                AddUserToRoles()
            End If

        End Sub

IsValid is copied straight from the DNN core code.  Adduser is a DNN property.

I then used CreateUser

        Public Function CreateUser() As UserCreateStatus

            User.Membership.Approved = False

            If User.Profile.LastName = Null.NullString Then
                User.Profile.LastName = User.LastName
            End If
            If User.Profile.FirstName = Null.NullString Then
                User.Profile.FirstName = User.FirstName
            End If
            User.Profile.SetProfileProperty("AFLTeam", ddlSportTeams.SelectedItem.Text)

            Return UserController.CreateUser(User)

        End Function

Then

        Private Sub UserCreateCompleted(ByVal CreateStatus As UserCreateStatus)
            Dim strMessage As String = ""

            Try
                If CreateStatus = UserCreateStatus.Success Then

'Do what you need here

And I have a new routine to add the roles to users wher I have a grid of services they can click a checkbox on.

       Private Sub AddUserToRoles()
            Dim objRoles As New RoleController

            Dim Selected As System.Web.UI.WebControls.CheckBox
            Dim intRow As Integer
            Dim DI As DataGridItem
            Dim TC As TableCell
            Dim RoleId As Integer

            For intRow = 0 To grdServices.Items.Count - 1
                DI = grdServices.Items.Item(intRow)
                TC = DI.Cells(0)
                Selected = TC.FindControl("chkService")
                If Selected.Checked = True Then
                    RoleId = CInt(DI.Cells(1).Text)
                    Dim objRole As RoleInfo = objRoles.GetRole(RoleId, PortalSettings.PortalId)
                    If objRole.IsPublic And objRole.ServiceFee = 0.0 Then
                        objRoles.UpdateUserRole(PortalId, User.UserID, RoleId, False)
                    End If
                Else
                    RoleId = CInt(DI.Cells(1).Text)
                    Dim objRole As RoleInfo = objRoles.GetUserRole(PortalId, User.UserID, RoleId)
                    If Not objRole Is Nothing Then
                        objRoles.DeleteUserRole(PortalId, User.UserID, RoleId)
                    End If
                End If
            Next
        End Sub

Uses all DNN objects and no custom SQL at all. :)  Works great too.