So you are looking for the results from AMB to T1, then add in T2, then add in T3, etc etc etc. Kind of like a stepped heating process where you reach steady state for a short period and then increase the temp for a defined time period.
You could set up a non-code combination where you are summing the total of the individual thermal load cases. You could also use the SMAX method as opposed to the SUM. SMAX would look for the absolute maximum, but then retain the original sign, so you would get the resultant maximum. If you were looking for the lowest value during all the conditions, and then the highest, you would have to set up two different combinations. This could be difficult though as the negative values are treated a little differently. You could easily get the absolute max and mins but you would lose the actual sign values, so it would have to be treated as a global value.