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For user crontabs (including root), you can do something like:
crontab -l -u user | cat - filename | crontab -u user -
where the file named "filename" contains items to append. You could also do text manipulation using sed or another tool in place of cat. You should use the crontab command instead of directly modifying the file.
A similar operation would be:
{ crontab -l -u user; echo 'crontab spec'; } | crontab -u user -
If you are modifying or creating system crontabs, those may be manipulated as you would ordinary text files. They are stored in the /etc/cron.d, /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly directories and in the files /etc/crontab and /etc/anacrontab.
Crontab files are simply text files and as such can be treated like any other text file. The purpose of the crontab command is to make editing crontab files safer. When edited through this command, the file is checked for errors and only saved if there are none.
crontab [path to file] can be used to specify a crontab stored in a file. Like crontab -e, this will only install the file if it is error free.
Therefore, a script can either directly write cron tab files, or write them to a temporary file and load them with the crontab [path to temp file] command. Writing directly saves having to write a temporary file, but it also avoids the safety check.