Transformation of an XML with visio file into PDF

Hello,

I'm completely new at transforming XML to PDF using FOP.
I have some XML document with visio files embedded.
I'd like to transform them into PDF.

Is there an object like <fo:external-graphic> for external documents such as .vsd?
Or Do I have no choice but to convert them into graphic 1rst?

Thanks in advance for your help.

Best regards,
Nalesk77