You can place slot holes in a pad, if you change the hole shape. L shaped hole...
Maybe two pads on top of each other?
When you think your software supports everything, connector manufacturers invent new problems...
Honestly, if it was up to me, I would just select a different part number and tell Hirose about it.
Also every connector footprint is simultaneously under and over defined in dimensions. The ones that you need are never there, but its filled with repeated info.