I have a cell that contains "01.08.2014", which is a date. I want to use a formula to turn it into "01.08.2014 00:00:00" which is supposed to be date and time.
Date/Time values are stored as a single number. The integer portion is the date (number of days since a reference date), and the decimal portion is the time (fraction of a day). That is the actual content of the cell if Excel stores it as date/time data. What you see displayed is just formatting to express it in human-readable form. Using string functions to parse the date components won't work because that is not what is actually in the cell.
You can format the contents any way you want. Custom formats give you a lot of flexibility. Or you can use functions to extract components of the date and time and concatenate them for a custom format. This link provides some explanation: http://www.excel-examples.com/microsoft-excel-date-time-functions.htm
EDIT: @Hans, When I wrote this, I missed your post that you had already solved your problem. I'll leave the answer in case it is useful to anyone else.