Driving the Options of a Given Package from Another Package or Class in LaTeX3/expl3

Driving the Options of a Given Package from Another Package or Class in
LaTeX3/expl3

I'm afraid I still don't get how I'm supposed to handle the communication
of options among packages in a LaTeX3/expl3 context. Let us say I have a
package pkg1 where I define an option opt_pkg1 that is set in the
document, but which is actually used just to drive the options
(opt1_pkg2,opt2_pkg2...) with which another package pkg2 is loaded from
the first (pkg1). What's a correct way to do this by using l3keys?