Is there a program to convert jar to midletpascal source code?
Most likely not, because midletpascal is not a popular product. But you can get the code, where possible, with a regular decompiler.
PS: are you sure that the game is written in pascal?
Tip - open the .jar file with an archiver.
For example WinRar'om.
And then already from Jav'y try something.