Haven't tried it myself, perhaps you could use FC33 S5TI_TM followed up by FC1 AD_DT_TM to add the times. You may need to use a dummy date and time as well because FC1s input parameters are DATE_AND_TIME PLUS TIME. FC40 TIM_S5TIM gets your answer back to S5Time.
What I can't see at the moment is how you get your DATE_AND_TIME result from FC1 back to the TIME format you need for FC40's input, because FC35 SUB_DT_TM also has the output in DATE_AND_TIME format, but I assume that it's in there somewhere. The FCs are all in the IEC Function Blocks Folder of the Standard Library.
I can't understand why Siemens don't offer Blocks for all the conversions you need. There's really no reason not to with modern CPUs and memory sizes. You have exactly the same round the houses going from INT to REAL and back again, where you've got to return via BCD, for heaven's sake!