How can I merge Linux local accounts and LDAP accounts? Can it be done?

The term 'merge' is vague. The PAM module you want is pam_ldap. http://wiki.debian.org/LDAP/PAM

You are changing the basic authentication/login for the system in question so it is easy to lock yourself out if you are not careful. Have a Live CD handy to back out bad changes.

With that warning out of the way, it's relatively easy to start tweaking PAM, you need ldap logging to debug things and some comfort working with LDAP in general to set it up successfully.

Once working, server replacements/upgrades are much easier. Especially if you move most services to LDAP backends.