Recommendation for Conference Session Bookings (After registration process)

We are running a conference where participants are registering and paying for 3, 2 or 1 day attendance as a set rate. We then need to have participants go through the 100+ sessions which are running at consecutive times over the 3 days and pick which session they wish to attend for each time slot. This should only be possible for participants who have already registered and paid for the registration. No additional fees should apply. We need to be able to track number of people booked into each session as the positions available are limited due to room sizes.

We are having issues finding a solution that will meet these needs. We are using Event Espresso Pro for our event registration process which is fine, but need a solution for this next part.

Can anyone recommend a timetable solution that will also allow participants to book into their sessions???