Android and iOS users could be easily imported to Pushwoosh from another provider. It is necessary to have a CSV file of subscribers with their push tokens and platforms identifier for correct migration.
On the other hand, Web users could not be imported due to technical limitations provided by the platform - all push provides use different payload formats as well as platform configuration, which makes import basically useless, as the userbase would be either invalidated or would be receiving blank/wrong content.
To migrate your push subscribers base to Pushwoosh, please contact us via Contact Us form and provide us with the sample of your userbase. We will check the formatting and migration possibility.