How to generate read data in CQRS from existing write database?

Currently we have an Asp.net application which uses SQL server for write and also read queries as a normal monolith application.

Now we want to move to CQRS.

In CQRS the read model is generated on basis of events.

But for the previous transactional data of my application, we do not have any events or logs.

So how can we generate read model in an NoSql database from data of our existing SQL server database.