Hi Olivia,
I'd love to learn more about the uniqueness of the data you're looking to "dynamically insert". The reason I ask is if the data you're using in the campaign is unique for each lead you're sending to, then mapping the data to a field in salesforce is really your only easy option (save for the suggestion from Kenny) .
It is possible to create Marketo-only fields, in other words, fields that only exist in Marketo, and never map to Salesforce, but this is generally not a great idea, as you may determine at some point you DO want that data in your CRM, and then it's a real mess to fix later.
Spreadsheet uploads are possible to do in Marketo, but these only work if they have a field to map to in your Marketo/CRM database. So even though you can upload a spreadsheet with all your custom values, if there isn't a field in your database to map that data to, it won't get imported, and you can't use it.
Hope this helps,
-Jeff