Yes, you can add a diff lock to the syncro tranny, but it takes some
costly machining.  There is a knock-out in the case for the diff. lock
mechanism, but it has to be machined to size and a groove has to be cut
out to fit a sealing rubber ring that keeps fluid from leaking out once
you put in the diff locking mechanism.  The actuator is about $165, not
to mention all the vacuum lines, switch, etc.  Best to bide your time
and look for a junker.

Speaking of junkers.  Today I saw an ad in Classified 2000 on the net
for a syncro in NY with a broken tranny for $800 total.  If you are in
the east, this may be a great parts vanagon.  Go for it.  here is the
info:
Syncro: 120K, needs body work, manual tranny broken, engine very good,
located in Valley Stream NY,  contact bsowacki@aol.com or (516)872-1802
eves.  Ask if it has the diff lock--and have it rebuilt