is there a way for Hibernate 2, when calling update(), to append to a
blob/text field, instead of replacing the value there was ?
if so, how ?

e.g. in MySQL syntax ;
update mytable set textfield = concat(textfield, 'value') where id = value

Doing this at the DB-level is more efficient (especially for blob/text that can hold much data) than pulling it up to the application, concatenize there, and saving it back

This is not possible as far as I know. Use a custom SQL query.