You can Parse
Xaml strings into elements as long as you have the required namespaces (or add them before parsing).
Then you can apply your parsed WPF element to a VisualBrush
Working Example:
string elementString = "<Grid xmlns=\"http://schemas.microsoft.com/winfx/2006/xaml/presentation\" > <Button /> </Grid>";
Visual result = XamlReader.Parse(elementString) as Visual;
VisualBrush myBrush = new VisualBrush(result);