Hi Simon,
you can do the same in SAP HANA - by using SQL directly.
For the graphical modelling tools you will find that not every option SQL provides is available.
This has to do with the fact, that many complex contructs are rather seldom used and also that something like your join condition will decrease the query performance.
If you want something like this to be executed real quickly you may consider using a generated column that is based on the LEFT() expression, just as you figured yourself.
Alternatively, if you really really only want to use the graphical tools, well, you can create a calculated column and use this one to join against.
I'd go with the calculated column, but hey, it's your database 
- Lars