I can find numerous resources that explain how to dynamically add a button to a sheet in a Excel VSTO Add-in project. Something like this should work:

Private Sub AddButtonToSheet(ByVal sheet As Excel.Worksheet)
    ' Add a button to the sheet
    Dim vstoSheet As Microsoft.Office.Tools.Excel.Worksheet = Globals.Factory.GetVstoObject(sheet)
    Dim button As Microsoft.Office.Tools.Excel.Controls.Button = vstoSheet.Controls.AddButton(50, 50, 100, 30, "myButton")

    ' Set button properties
    button.Text = "Click Me"
    AddHandler button.Click, AddressOf Button_Click
End Sub

Unfortunately, I’m not able to get the the call to Globals.Factory.GetVstoObject(sheet) to work in a VSTO Workbook project (it returns Nothing).

Is it possible to create buttons at runtime using VSTO Workbooks and if so, how?