Hi and thank you for posting! Welcome to the Avenza Maps app!
To import a custom import map in the app, you should first make sure it is a georeferenced map in a supported map file format. Georeferencing can be done in GIS software such as our MAPublisher and Geographic Imager software. For more information on file formats and importing maps the following links are offered: Importing maps from custom locations on Android and Importing maps from custom locations for iOS.
Please let me know if you have any questions! Regards.