Hi Ryan,
Yes, you can purchase Personal Use in the Esri Store using your existing Esri account. When prompted to select a customer number, instead of selecting your business account, choose the option to create a new customer number. Nex, simply indicate you are purchasing for yourself as an individual. This will create a new customer number for you, personally.
If your existing Esri account is already associated with an ArcGIS Online Organization (arcgis.com), you will need to create a different/new Esri Account, associated with a different email address, to activate your personal ArcGIS Online organization.
For additional information about purchasing Personal Use, feel free to contact us at storehelp@esri.com.
Thanks!
Tina