Adding characters and numbers at the end of each cell

Hi
How do you add :00 at the end of each cell in a column in Excel 2003 (or openoffice)
I do a lot of video editing and when reviewing the videos in order to give me editing instructions, clients write down the start and end times of each clip in Excel documents.

Below is a screenshot of a typical document from a customer.

Excel_How-to.jpg

Column B and C are the start and end of each clip, directly copied from the Adobe Premiere.
As you can see, there are more number
00:03:12:10 means 3 minutes 12 seconds and 10 frames
I tell clients to disregard the last 2 digits as they symbolise the frames, but I would love to be able to add ":00" at the end of each cell in columns D and E.
That way, I would be able to copy and paste to the exact location on my video timeline. As it stands, if you look at column D8 for instance, where the client typed 6:23 for 6min_23sec, I ideally would be able to automatically change it to 6:23:00 otherwise pasting 6:23 on my Adobe Premiere timeline will bring me to 6seconds_23frames (instead of 6min_23sec_00frames).
I hope that makes sense