How can I alter the fill factor of an index outside of an ALTER INDEX command?
Check your DB Server (or instance) options, it will set every index based on the fill factor you specified.

sp_configure 'show advanced options', 1
GO
RECONFIGURE
GO
sp_configure 'fill factor', x --set the fill factor value here, such as 70 for 70%
GO
RECONFIGURE
GO
sp_configure 'show advanced options', 0
GO
RECONFIGURE
GO


edit:

Do note, it will not actually rebuild or alter existing indexes.  It will fulfill your request that all future index rebuilds will now have this fill factor, unless specified in the rebuild index command.  Anything specified in the rebuild command will override this option.