I realise that Preform does not let you manually modify the support structure. However if you could export the file as an STL those with the appropriate software can delete/add supports as needed after Preform has had a go.
Good day Josh,
before they include this function in Preform, you can always use the small utility here: http://wp.me/p3gI7M-90
It’s only command line, so not very integrated, and while it exports each object separately with its support structure, the rotation and scaling are not included.
I use the b9creator software (free download) to place custom supports and it works great. You have a lot of control of the supports, location, angles, thickness, etc. Then it exports the file to STL and import it into the Preform for printing.