Yes you can add the eMail adress from your GApps account via
Settings > Accounts & Synchronisation > Add Account
Then enter your credentials and you can access your accounts eMail, contacts and calendar without automatically sharing information from your private calendar with your colleagues.
But you need to watch out which account you use creating new calendar entries from now on.
After that you need to open your calendar application and press Menu > Calendar > Select the calendars you want to see.
If you don't want to see your work eMails you can easily decide which items you want to synchronize with your device.
Merging the calendars is not really necessary since you can view many different calendars in the calendar application.