Can an insurance company that issued a general liability policy send a bill pursuant to an audit and request additional premiums on a policy that expired 6 months earlier?

Yes many Liability Policy are subject to audits which occur on policies after the term dates. The reason for this is that the original policy premium is generally based on estimates of of the exposure basis (i.e. payrolls, receipts, etc.) The insurance company is entitled to audit at the end of the policy term to adjust the premium to reflect the actual exposures.