Many applications support webhooks, which allow you to send or receive data via custom URLs. You can use Zapier's webhook functionality to create a custom integration between the unsupported app and other apps. This goes for both trigger steps in your zap, and search and action steps.
Check the documentation of the app you want to integrate to see if it supports webhooks.
If the app you want to connect to has an open API, you can create a custom integration using Zapier Platform. You'll need to have some knowledge of APIs and how to work with them, and you'll need to refer to the app's API documentation to configure the integration properly.
If the app you want to integrate sends emails with structured data, you can use Zapier's Email Parser or another email parsing service to extract the data and trigger a Zap.
This method is useful when the app doesn't support webhooks or APIs but does send email notifications.
If the unsupported app allows for importing and exporting data via CSV or other file formats, you can set up a semi-automated process by periodically exporting data from the app to a spreadsheet (Google Sheets, Airtable, Smartsheet) and then using Zapier to process and import the data into the target app.