Combine datetime + string to make a proper datetime
This will also deal with values &lt; 10 AM that don't have all 4 digits. (Also that column should probably be CHAR(4) instead of VARCHAR(5) - or better yet, stop separating these and store a proper datetime value in the first place).

DECLARE @t TABLE(ColumnA DATETIME, ColumnB VARCHAR(5));

INSERT @t VALUES('20130701', '1330'),('20130701', '930');

SELECT CONVERT(DATETIME, CONVERT(CHAR(8), ColumnA, 112) 
    + ' ' + LEFT(RIGHT('0' + ColumnB, 4),2) 
    + ':' + RIGHT(ColumnB, 2))
FROM @t;