Though this is not really an answer to why you can't JOIN to the master file created on the LDAP adapter (I'll leave that up to the experts), there may be a way around it.
As it seems as if you can actually query it, why not do that, HOLD the results and then join to that HOLD from your "users" table?
That's how I would do it anyway. I am always skeptical about JOINing disparate data sources even if WebFOCUS syntax allows me to. I prefer the read1/hold1-read2/hold2-readn/holdn-join/match approach which has proven to be easier to debug and tune than trying to put all together in a single structure.
As the sources are disparate, WebFOCUS will still need to break it in separate requests anyway (or worse, looping in some cases) in order to get you the results.
TABLE FILE LDAP_AD_MASTER
ON TABLE HOLD AS HLDAPINF FORMAT FOCUS INDEX USER
Then JOIN to/from HLDAPINF on HLDAPINF.USER as you see fit.
Hope that helps!
Prod/Dev: WF Server 8008/Win 2008 - WF Client 8008/Win 2008 - Dev. Studio: 8008/Windows 7 - DBMS: Oracle 11g Rel 2
Test: Dev. Studio 8008 /Windows 7 (Local) Output:HTML, EXL2K.