SOLVED. 
It was simple but the documentation was misleading…
We could not find a way to create a virtual AD container to be used as a base DN for the user search but there is another way:
in the definition of the Directory Service you have the section “Advanced Object Filters” and options “User Object Filter” and “Group Object
Filter”.
The problem is you that CANNOT USE A COMPLEX QUERY such as:
(&(objectCategory=CN=Person,CN=Schema,CN=Configuration,DC=EXAMPLE,DC=LOCAL)(memberOf=CN=D10927.BPMS_LOGIN,OU=Technical,OU=BPMS,OU=SECGRP,OU=APP,OU=ORG,DC=EXAMPLE,DC=LOCAL))
You have to simplify it to:
memberOf=CN=D10927.BPMS_LOGIN,OU=Technical,OU=BPMS,OU=SECGRP,OU=APP,OU=ORG,DC=EXAMPLE,DC=LOCAL
SO, no parenthesis, single expression.
The restriction to the object category person is assured by the User Attributes/User Object Class afterwards.
Thanks to all the people who contributed with suggestions. 
Happy hacking!
#webMethods#MWS-CAF-Task-Engine#webMethods-BPMS