I am in the same boat (ship) I went from CA to VA in a short time using only DOFFs. I think that you must complete a few of the missions in order to unlock that mission. I was looking for a list when I saw your post. I am thinking it may just be a few of the first contact missions that opens up a new set. I will let you know ASAP.
Because of the forced linear mission progression now you have to complete or click skip on every mission that comes before the mission that has you go b'tran. It won't even show up until you have either completed or skip the other missions.
You have to have completed or pressed skip on every mission on the Klingon front, Romulan Front and cardassian Front before the "Report to Gamma Orionis" mission will show up.
Press J and click on the episode tab. On the left you will see the 6 different fronts(Klingon, romulan, cardassian, borg, breen, undine). click on the klingon one and it will show you all the missions on the klingon front. Click skip on any that you have not done then do the same for the romulan and the cardassian and after you have clicked skip on all of them click on the borg and it should then let you accept the report to gamma orionis mission.
you do NOT need to get access to do the STFs though as the PvE queue is accessable everywhere. An advantage now to waiting till max on the storyline missions now is that the majority of the rewards are leveled to your current level.