I got some information from the author of Rserve. The variable changed in Rserve will be available to RSclient connected after the changing. In particular I got these codes working.
$ ~/bin/R CMD Rserve --RS-enable-control
$ ~/bin/R
library(RSclient);
c=RS.connect();
RS.server.eval(c,"xx<-1");
## [1] TRUE
RS.close(c)
## NULL
c1=RS.connect();
RS.eval(c1,quote(yy<-xx));
## [1] 1
quit()