Sure, just add it to the document library which will do the trick.
With v15 of the Echosign app, you can reference Echosign library document directly. This will work best if you added the tags via drag and drop in Echosign instead of text tags.
If you upgrade from a previous version of the app, then you need to enable this feature. See the upgrade guide for more details. The upgrade guide is linked from the customization guide.
If you have used text tags to add fields to your documents, then you can indeed simply do as Jat says and add the document with the text tags to the Salesforce document library.