You might do something like this:
var oldBinding = BindingOperations.GetBinding(element, UIElement.VisibilityProperty);
var newBinding = new MultiBinding { Converter = ... };
var firstBinding = new Binding
{
Path = oldBinding.Path,
Source = oldBinding.Source // or RelativeSource or ElementName
};
var secondBinding = new Binding
{
...
};
newBinding.Bindings.Add(firstBinding);
newBinding.Bindings.Add(secondBinding);
BindingOperations.SetBinding(element, UIElement.VisibilityProperty, newBinding);