Hi Csquared,
Thanks for your question. My name is Josh Wells and I am a support engineer on the Microsoft Dynamics CRM support team.
There's not going to be an easy way to do this. You can't create a Marketing List from an Excel spreadsheet. You really would need to do it through the SDK or through the suggestion that Muhammad had about using AF. You could create a new option set for the entity you are working with and call that something like Marketing List Option. Then export the records you wanted to add to the marketing list with the option to re-import. Update those records to all have the same option set value. Then reimport those records. You could also possibly do something like a bulk update.
There has been a request that this sort of functionality be added into the system. You can find that request here:
connect.microsoft.com/.../ability-to-import-marketing-list-members-through-excel-ws-4294
The more votes we can get on this, the higher on the radar it goes to the development team. Please go to the Connect site above and vote this option up.
Let us know if you have any additional questions.