You're looking at a simnple two-step process:
1) Convert the string to a date
2) Convert the date to a datetime
This can be done in a single line of code:
new_dt = dhms(input(date_str, ddmmyy10.), 0,0,0);
Now you have a datetime. But I do not know a predefined format for that so you may have to roll your own. Have a look at this how to do that.
EDIT: This would be how to do the custom format:
proc format;
picture mydt other='%0Y-%0m-%0d : %0H:%0M:%0S' (datatype=datetime);
run;