Is it possible to make improvements to a service that does not have an open API?

Question

API

Hello!
On our Internet projects, we use the E-autopay CRM system for billing (
but this system does not have an open API interface, and now we need to slightly change the billing process, namely, add the ability to pay from the user's personal balance. (This functionality is not in the basic configuration )
You can see on the screenshot how it should look like in the end ...

So the question is, is it possible to make any changes to this service without the API or not?

Answer 1

If their sources are not encrypted and this is allowed by the license agreement (or you are parallel to the license agreement) , then you can.
Otherwise - it is impossible.