How to define + operator for strings

["Andrus" <eetasoft@xxxxxxxxx>]

I want to create portable code which runs in other dbms without 
modification.

Unfortunately this other dbms uses + for string concatenation and has no way 
to define operators.

How to define + operator as alias of || operator for strings  so I can use

SELECT firstname+ ' '+ lastname
...

in Postgres

Andrus.