Private Sub form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Dim form2add As New Form2
    form2add.TopLevel = False
    SplitContainer1.Panel2.Controls.Add(form2add)
    form2add.Show()

    Dim form3add As New Form3
    form3add.TopLevel = False
    SplitContainer1.Panel2.Controls.Add(form3add)
    form3add.Show()

    Dim form4add As New Form4
    form4add.TopLevel = False
    SplitContainer1.Panel2.Controls.Add(form4add)
    form4add.Show()
End Sub

Form2.ListBox1.Items.Add("Texto a ser adicionado")

Friend WithEvents form2add As Form
Private Sub form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
     form2add = New Form2() ' Criado a referência semi-pública do objeto '
     form2add.TopLevel = False
     SplitContainer1.Panel2.Controls.Add(form2add)
     form2add.Show()
     ...

form2add.ListBox1.Items.Add("Texto a ser adicionado")