Hi Eric,
You can do this as follows:
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Create a Marketo-only field "append this"
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Change data value, attribute "append this", new value "Product 3"
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Change data value, attribute "super-cool-field-name", new value: {{lead.super-cool-field-name}}; {{lead.append this}}
The result will be "Product 1; Product 2; Product 3", which will show up as multiple values in SFDC.
And yes, test this carefully before going into production. A typo could cause data loss.
Best,
Jep