IF_SWF_IFS_PARAMETER_CONTAINER and multi-line param

I am trying to set an event parameter using interface method IF_SWF_IFS_PARAMETER_CONTAINER->SET, for example:

CALL METHOD event_container->set

EXPORTING

name = 'Number'

value = '2'

However, the parameter is a multi-line parameter. So if I execute the code above twice, it just overwrites the parameter instead of appending.

How can I append a multi-line parameter using this interface?