no
and this holds for most things that are sold directly & on steam
1 or other not both or mix'n'match (annoyingly)
I can't say for sure, but I thought I read somewhere that what is bought on Steam only works on the Steam version. Of course I can be wrong, but I wouldn't take the chance. Let's hope for an official answer.
santhoranb16_ESO wrote: »You can only buy a DLC on steam if you own the base game on steam.
You can link your account to steam however if that steam acc is already linked to an(other) existing eso account you will automatically buy to that one.