First, usually the order of PDF box sizes are Media>Crop>Bleed>Trim>Art, so you would usually have trim box that is smaller than crop box, so the statement should be: cropbox = trimbox + 2.5 mm
In any case, you can do this with the resize node.
1. Set the Resize to be applied to Crop Box, and make it to be To Size.
2. Set the Content Scaling to be None.
3. For Width, click on the chain symbol right of the field and set it to a Calculated Value.
4. Then in the field add the calculation, "$FFwfDoc.trimWidth$ + 500".
Note, on "metric" systems the numbers are 100th of millimetres, so 500 translates to 5 mm. Also note you need to use twice the value you wish the box width to be, as it's divided equally on both sides.
5. Do the steps 3 and 4 for the Height field and use "$FFwfDoc.trimHeight$ + 500" as the calculated value.
You can use other PDF boxes or values from the Job File in the Calculated Values field - see the system help, Appendix, Job File for the list of Variables available.
