You can export your content from Notion into Boost Note one doc or all at once, here's how:
Exporting individual Notion doc into Boost Note
In Notion, select the doc you wish to export and click "Export"
Export file as Markdown&CSV or HTML
Upload files in the Boost Note
Exporting Notion docs in bulk into Boost Note
Go to Settings & Members in the left-hand menu and open Settings
Scroll down to Export, and click Export all workspace content
Export file as Markdown&CSV or HTML
Upload files in the Boost Note