Model-driven apps don't natively use the connectors that canvas apps use, but there are options.
The biggest thing, before you integrate your on premise data into CDS is make a plan. if you have 10 on premise systems, likely they have similar data in them. You probably have things like contact data in multiple systems. having 10 different two-way integrations is a recipe for a lot of complications and potential data corruption.
Whenever I do an integration with multiple legacy systems, I want to do a single data integration rather than multiple if possible. create a data warehouse or use a data lake and bring your data together--deduplicate if there are multiple copies of the same record. if you have contact data in multiple systems, you need to set rules about which wins in the case there are multiple.
so what are your integration options? Depends on what you want to do with the data. Is it actionable data, meaning the user must make updates to it, relate activities and notes to it, etc, or is it reference data that is mainly read only?
If it is actionable data, you will want to integrate the data into the CDS. You can use Azure Data Factory, SSIS, etc and create a two-way data integration. I recommend the Kingswaysoft (www.kingswaysoft.com) adapter for SSIS.
If it doesn't need to be two-way, you can use data flows https://docs.microsoft.com/en-us/powerapps/maker/common-data-service/create-and-use-dataflows
these are standard one-way integration processes that use PowerQuery.
Another option, if the data is related to primary records in CDS that you might want to consider is embedded canvas apps. this lets you embed a canvas app in model-driven forms. https://docs.microsoft.com/en-us/powerapps/maker/model-driven-apps/embed-canvas-app-in-form
For example, if you have customer data in another system, you could embed a canvas app on the account form using the SQL server connector and display data from SQL server in your model driven app.
Another option, if you have customer data in multiple places, is to use Customer Insights. https://dynamics.microsoft.com/en-us/ai/customer-insights/. Customer Insights is a Customer Data Platform (CDP) that can bring data from multiple systems together and is integrated with CDs and the power platform.
