The input mapping in the next task is the easiest solution 
Or, wrapping the multi-instance activity in an embedded-subprocess that do the transformation in the output mapping.
Just to demonstrate how the solution could look like using an output mapping on the multi-instance activity.
Given that the workflow instance has the following variable (e.g. passed in as variable on creation):
outputCollection: {"results":[]}
When the job is completed with a variable “outputElement” then the result can be collected using the following output mapping on the multi-instance activity:
source: =append(outputCollection.results, outputElement)
target: outputCollection.results
It appends the output to the result list and updates the property “results” in the “outputCollection” variable.