Adding unsigned 256 bit integers in PostgreSQL

I was wondering if there would be any way to do the following in PostgreSQL: UPDATE cryptotable SET work = work + 'some big hexadecimal number' where work is an unsigned 256 bit number. Right now my column is a character varying(64) column (hexadecimal representation) but I would be happy to switch to another data type if it lets me do the operation above. If it's not possible with vanilla PostgreSQL, are there extensions that could help me? (I also [posted this to pgsql-hackers](http://www.postgresql.org/message-id/CALwxDuETY1_jzBSQC+nR-uBY0eZHhH5poTMo0KCCi14Z4RtCtQ@mail.gmail.com) so check out the thread there for more ideas.)