If the old application has the data in a database, you could use our various tools to read the data as an external data source and load it into the clipboard. From there, it's a property to display, just like any other.
It's hard to get into specifics without knowing how your old application is designed. If you have tables of data in a database, you should be able to query them, like any other DB table. If they are stored in some other format, you may need to do some transitional work. The good news is that Pega can read flat files, XML files, or any other number of industry standard ways for systems to pass data. You've got options. The biggest limitation would be on the legacy side.
For your requirement, I assume that your data is stored in some DB and if it is so, then for creating work objects, you need to write some activities explicitly which can connect to the DB and get data row-by-row and creates a Work object for each row. We had somewhat related requirement for our application and we used SQL DB to which we connected from PEGA end and migrated the data from previous application to current application.
But for your data model changes, I feel that there must be some Pega Data classes to be created on our end and make sure that they have a mapping with some table on the external DB end.