Hi @johannes-m 
No problem at all, happy to help.
It’s not something I’ve tried before, but it might be worth exploring the use of “OBJECT_PROPERTY” (within supportedValueTypes definition). As described within HubSpot Developer documentation:
OBJECT_PROPERTY**:** the user can select a property from the enrolled object or an output from a previous action to use as the value of the field.
If this doesn’t fit the bill, you might need to consider HubSpot custom code workflow action, although this would be HubSpot Portal-specific (rather than supplied via an installed app).
I hope this proves useful. Please let me know if you have any follow-up questions.