Hey there, Cmlee5150.
Welcome to the Apple Support Communities! Thanks for your question about using the contacts from Outlook in Messages. We'd be glad to see what we can do to help you out.
Messages should have access to any entries stored within the Contacts app on your device. If you go to Contacts > Preferences > Accounts, do you see that account added there? If not, look over Add contacts from iCloud, Google, and more to Contacts on Mac for the steps to get that account added.
Hopefully that'll get you helped out.
Take care.