Alias to a type

Hey,

Is it possible to create a new type as an alias to a pre-defined type? I 
use "USERID varchar(20)" in almost every table I have I'd like to make 
an alias for that type eg.

<pseudo-sql>  create type myschema.useridtype as varchar(20);
</pseudo-sql>

I might have to alter the type some day and this way I'd have to just 
re-define the alias. Can this be achieved by creating a new type with 
CREATE TYPE? What are the input/output functions in the CREATE TYPE 
definition?

Thanks.


-veikko