Is possible create custom conversion from mysql database to IPB database with custom fields?

Well, it is possible to move data from one data source to the IPS MySQL database as the final destination. Whether or not the IPS Pages database feature has the necessary featureset to support whatever it is you're converting remains to be seen.

You can create a custom conversion, or use an ETL tool independent from IPS to move the data from one place to another, as is appropriate, and make any transformations that might be necessary.