Is there any way to forward all spam mail to only one account (aka. allofspam@company.com)?
The sorting to a user's junkmail folder is done by /etc/e-smith/templates-user/.qmail-junkmail/10deliver_to_junkmail
You should be able to create an equivalent file in templates-user-custom to override the default behaviour.
As usual, as this is a custom template, all bets are off for future upgrades.
BTW: This might be worth raising as a New Feature Request in SME Server Future once you have it working.