I am trying to do this too and when my smart campaign is part of a webinar program in one workspace when I add flow step "Add to Engagement Program" none of my engagagment programs show in pull down list.
and visa versa if I try to do within engagement program saying if member of program X add to engagement program I see engagement programs fine but smart list doesn't see program i want members of.
This is really a problem for our use case.
Any workaround found?
The workaround is to rely on entities that are accessible from all workspaces. Database fields e.g. or shared lists.
Your webinar program in Workspace A could set a value in a custom field "Engagement Program Entry", and your Engagement Program in Workspace B could pick that data value change up.
List Membership in lists that are shared across workspaces could do the same.