[quote]I don't have Forsaken Addon on Blizzard.net. Will I get it if I pre-order Shadowkeep on Steam? Or it will be free?[/quote] Did you read what is included in Shadowkeep? Does it say anything about Forsaken or the Forsaken Annual Pass being included? No, it does not. [quote]Or I can buy it now on Blizzard.net, complete and all the data will be transferred on 1st October?[/quote] If you are asking can you buy the Forsaken bundle now and it will be transferred to Steam, then yes.
It is not (and likely will never be) free, nor is it included in any current Shadowkeep bundles. If you want it, you’ll need to buy it from blizzard now, or wait for the transfer to steam then buy on steam.