In Vixie cron, which is possibly the most common, you can do this almost exactly like a shell script.
0 0 * * * doathing.sh $VARIABLE
The man page says:
An active line in a crontab will be either an environment setting or a cron command. An environment setting is of the form,
name = value
where the spaces around the equal-sign (=) are optional, and any subsequent non-leading spaces in value will be part of the value assigned
to name. The value string may be placed in quotes (single or double, but matching) to preserve leading or trailing blanks. The name
string may also be placed in quote (single or double, but matching) to preserve leading, trailing or inner blanks.
You can tell if you have Vixie cron by checking the man page for
crontab; the author will be Paul Vixie. Different crons may or may not support this (BusyBox’s cron for example, does not), in which case your best option is to wrap your command in a shell script and run that script from cron instead. In fact, this is a good thing to do for anything complicated.