How can clients use already existing credits with a previous accounting system?

Are you looking to integrate your form with your system? 

If so, you may check our apps and find if we have an integration with them, here is the link: https://www.jotform.com/apps

In case you cannot find it on that page, we can open a feature request too. 

If I misunderstood your question, please provide us more details. 

Thanks.