Hi, Mike,
Yes, it is. As a very basic example, here's what you would do to add a textbox at runtime...
System.Windows.Forms.TextBox oText = new System.Windows.Forms.TextBox();
this.Controls.Add(oText);
oText.Visible = true;
oText.Top = 5;
In that example, I'm just using the base text control. If you've subclassed your textbox, you'd want to specify your textbox class instead.
You can also add a series of controls by using this.Controls.AddRange()
Actually, if you look at the Winform Designer generated code, that should give you ideas as to how it's done, because the controls you drop onto the form at designtime are added at runtime using 'this.Controls.Add'.
Hope that helps...
Kevin