voffset=\dimexpr 297mm-18mm-25mm\relax]
{\externalfigure[file.pdf][page=3]}%
</texcode>
To clip a picture to a circle (from the MetaFun manual):
<texcode>
\startMPclip{circleclip}
clip currentpicture to fullcircle shifted (.5,.5)
xscaled \width yscaled \height ;
\stopMPclip
\clip[nx=1,ny=1,mp=circleclip]{\externalfigure[hacker.png]}
</texcode>