Hello,
You can just use the php class (found in the "3rd party software" part of the forum) to interface with DirectAdmin. To create email accounts, you'll use:
/CMD_EMAIL_POP with method POST (might need to modify the php class to handle "POST")
action=create
domain=domain.com
user=emailuser
passwd=the pass
passwd2=the pass
create=Create
Needless to say, some programming knowledege is required.
John