I discussed with my co-work before for similar question, and the conclusion is that there is no way to do this in a single standard sql query(no external UDF or stored proc). And should be fair easy and efficient by using an application. (for example StringBuffer class in java).
Anyway, with the latest XML functions, it is doable with XMLAGG and XMLQUERY functions, such as(run on db2/zos):
CREATE TABLE TT
INSERT INTO TT VALUES('welcome');
INSERT INTO TT VALUES('udb');
INSERT INTO TT VALUES('db2base');
XMLAGG(XMLELEMENT(NAME "COL1", COL1))) )
AS CLOB(50) EXCLUDING XMLDECLARATION)