Actually, there is a way to semi-control the UP Agenda but it can be tedious and time consuming to get the issue you are interested in voted on and, hopefully, get the decision you wanted on that issue.
I found that the only issue I really care about is getting all the ET races to share their tech with me through the voting of the United Planets. I came across this method of "unrandomizing" the issue: When the AI gives you the warning that the UP meets the following month SAVE THE GAME! You want to save the game with minimal other events going on on the following turn as they slow the process down (no constructors reaching bases, no wars if possible, etc.).
Hit the NEXT TURN button and voila you have your random issue to vote on. BUT IF IT IS NOT THE ONE I WANT I just reload the SAVED GAME and do it over and over again until the Tech Share issue gets voted on AND gets the decision I want. Sometimes this becomes impossible if one or two ET's have too much influence and override all other voters.
But when it works YOU HAVE EFFECTIVELY ELIMINATED THE RANDOMNESS OF THE UP ISSUES GETTING VOTED ON!!!
Just remember sometimes it takes a very, very, very many tries to get what you want (especially me as it seems the Tech Share issue is one of the least selected to vote upon).
If that is the UP agenda you want, you can further stack the odds by selling tech to the minors for all of their influence. You can even sell your tech to the other Major powers for influence (you are going to lose it anyway)...