First, let's assume the data table is currently set up to accommodate T4:0 to T4:10, for example. Now you want to add T4:11. That's increasing the size of the data table which I believe cannot be done "online". Actual timer usage has nothing to do with this either.
However, in the above example, let's say T4:5 wasn't used yet. You should be able to add a rung containing that timer in an online edit. (I will assume you know how to do an online edit.)