The easiest way is to export the list as a .csv file and import it directly into Marketo. If you want to get fancy, you can do this same thing with our API. Here is a description of how to use it: Developers.marketo.com
The solutions are multiple and depends primary of the capabilities of your proprietary software. It can range from an old fashioned export from the software and import into Marketo to an API connector that sync's the 2 products, or using a third party data movement software such as Talend, Informatica or Bedrock Data.