Hi Hayim,
I don't think full outer join is supported in any of the HANA models. And as you mentioned you may want to go with UNION.
Do have a look on the below:
SAP HANA: Is Full Outer Join not supported through Graphical models?
Thinking in HANA - Part 1: Set Operators
SAP HANA: Achieving Set operations using Joins
Regards,
Krishna Tangudu