In a spare row or column of your spreadsheet that lines up with the existing data, enter the following formula:
Where A1 is the cell with the time in seconds. That function takes hours, minutes, and seconds as parameters and converts it into OpenOffice's regular time format.
Then, copy or drag to autofill the rest of the cells that are necessary to perform the calculation on all the relevant data. Now, highlight all the newly calculated values, right-click and choose Cut or press CTRL+X, then highlight the old values and right-click and choose Paste Special. Uncheck Paste all and then uncheck everything but Date & time and click okay.
Finally, right click on the whole set of cells, choose Format Cells, select Time under Category and select whichever format you desire.