Due to recent steam policies- You cannot put a pre-order on DLC unless a release date is given.
now, with the Mig29 ED actually put the release date as the very last day of september- and those with a keen eye would notice, that before the pre-order opened on steam, it was marked as either "TBD" or coming soon. But as soon as pre-orders open there has to be a release date.
What I have seen 3rd parties do- is to have the product release on steam on launch day- and keep the pre-order discount applied on the steam store for a week or two, HB did it with the Phantom since they avoided opening pre-orders on steam, so they offered the equivalent discount amount for a few weeks post- release.
GUCCI's post in When can we pre-order C-130J on Steam? was marked as the answer