DB2 - Inserting a timestamp operation result in a timestamp field
Following sequence of statements worked fine in Db2 (for LUW)

create table mschema.MDUMMY (k varchar(10), n int, t timestamp)

INSERT INTO mschema.MDUMMY (k, n, t) VALUES (   '70',   70,   (SELECT
TIMESTAMP('1965-07-27-15.30.00') + 59 SECONDS FROM SYSIBM.SYSDUMMY1) 
);

SELECT * FROM mschema.MDUMMY


I just copied the second statement from your question. 
This means you made some other mistake running it or you are maybe not running it on Db2 LUW - this is why it is always useful to specify the OS.