You can do something like this :-
crontab -l > /tmp/origcron
echo "00 10 * * /usr/bin/ls -l > /tmp/list.log 2>&1 " >> /tmp/origcron
crontab /tmp/origcron
(just an example ls -l, you can call your program)
If you require running different jobs when required at different times , you can us the at command and specify when to run the job e.g.
at 5 pm
(run a script) /home/script.sh
HTH