Hi Oliver,
It is possible. You need to:
1) Create a screen and add "Assignee" field to it
2) Attach this screen to the transition you want to select an assignee at
3) Add a validator to that transition that would require a value to be provided to "Assignee" field during the tansition.
Please note that 'Field required validator' is only available off the shelf in JSD Cloud. If you are using Server then you'll need a 3rd party addon like JMWE to have it.
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