[sdiy] Possible to Hard-Sync R. Schmitz CMOS VCO???

[cs e]

How would someone go about adding hard-sync to Rene Schmitz's CMOS VCO
(as seen here, in beautiful detail:
http://www.uni-bonn.de/~uzs159/vco4069.html   ?

I would appreciate to read anyone's process/analysis of determining
the possibility of adding sync; and of course I would love to read how
to do it ;-)

Thanks