Hi Jörg,
Marketo API is easily accessible (Marketo REST APIs: Lead, List, Activity, Campaign Objects » Marketo Developers ) but in order to push data from Google docs, this will either be a development within Google docs (I just do not know if this is possible) or a third party tool that extracts data from Google docs and pushes it to Marketo.
With regards to the details, you should talk to a Google Docs specialist, which I am not 
and I am not sure this community is the best place to find some either 
.
-Greg
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