Migrating subscribers from a different tool

We have been using another tool for push notifications for more than a year and I want to know if we will be able to migrate our subscribers to Webpushr if we choose to switch.

Yes, you will be able to. Please take a look at the following two resources:

https://forum.webpushr.com/t/how-to-track-migration-transfer-of-subscribers-from-onesignal-to-webpushr/22

https://docs.webpushr.com/transfer-subscriber-data