Pretty sure that it's not possible.
Let me clarify on this, real fast. This confusion comes up a lot.
For every copy of A Realm Reborn (aka "Starter Edition", or the base game) you add to the account, you will get 30 free days of playtime. The expansion packs do not give free playtime.
Every FFXIV account may currently have a single license attached for each of the currently supported versions. This means you can add one copy of A Realm Reborn for Windows, one for PS4, and one for Mac. If you add all three to your account, you will get 90 days of free play time. But you cannot add a license more than once per platform. You must also buy each expansions on the respective platform you wish to play them on.
For example, this is my account page:
In my case, I can log into the game on PS4, but I cannot go into Heavensward or Stormblood areas.
However, there is a distinction that people should be aware of: What Square Enix calls "Windows" can actually refer to the standalone version, or it can also refer to the version on Steam. You may only have
ONE Windows version on your account. You may not (currently) mix and match the Steam and Non-Steam versions.
Related to this, if you buy the game on Steam, you are locked into Steam. Which means you
must buy all future expansions on Steam. And expansions purchased on Steam will not work on the Non-Steam version. On the flipside, you can pay for your subscription through Steam if you buy it there. For some people, the regional pricing is better too.
TLDR: You need a license for each platform you want to play on and each expansion per platform. "Windows" can be either Steam or Non-Steam, but not both. You get 30 days free for each license of the base game added.