How can I account for package trace length in KiCAD?
I haven't tried this, but I found this inside the documentation of the footprint editor:

If you edit the pad properties a field shows up marked as "Pad to die length". I guess this is what you are looking for. So edit the package of the chip and adjust all the pads with the corresponding numbers from your datasheet.

 picture taken from here.

You might want to try that with two pads first before editing hundreds of them.