Solution : https://service.sap.com/sap/support/notes/602448 (SAP Service marketplace login required)
Key words : 
import parameters  parameter    type  reference type  default vl  opt, export parameters  parameter    type   reference type   pass, creating function module create function module co2o_act_set_ext_proc_data, se37 function group co2o short text, 'x' states'internally processed activity / external processing, required external processing data 2, required external processing data, checks external processing data, type 'external processing', 'internally processed activity'
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