With the majority of companies, a prior insurance discount for auto coverage is based on being previously insured under an auto policy. Unfortunately most companies won't consider an existing motorcycle policy
to be sufficient to qualify for a continuous insurance discount. It may be possible that the company insuring your motorcycle would honor the discount if you choose to take out an auto policy with them, but there is no guaranty of that happening. Bundling may still be your best bet since most companies offer a discount for having multiple policies with the same provider. As always when shopping for any type of policy, comparing rates
with as many companies as possible is the only way to find the best policy.