Sequence - Use calculated start value?

I need to use a calculated start value for some sequences. eg:

CREATE SEQUENCE tblco_cntcoid_key start (SELECT (MAX(cntcoid) + 100)
FROM tblco) increment 1 maxvalue 2147483647 minvalue 1 cache 1;

when I try this I get:
ERROR: parser: parse error at or near (

I am migrating an MS Access DB to postgresql and need to retain the
current value of the id columns (er cntcoid), but when I insert new
values I want the cntcoid to start at a number greater than the last one
in the table. When I use a normal sequence or serial type for the id,
any new records are inserted starting at id 1.

Anyone know how to do this?

Thanks

--
Ron
Syscor R&D