I would like to know if already exist a converter for take gui program (.P) to convert quickly in Telerik UI or DevExpress UI?
Currently there is no such thing as a conversion process, however you can find different strategies.
Use the proxy generator to generate libraries that understand the new tools for example.
The main point you will have is whether or not the process was operating in layer logic, messaging and the type of database connection.
If your process operates in layers and messaging logic, the code can easily be separated and place the corresponding part in the application server and use the new tools centered in the front, otherwise you will need to separate this logic and generate new procedures.
An important precedent for this type of process is webclient which already operated with layer logic and messaging.
Remember that the main problem that you will face in a legacy system is the form of connection to the database, in most cases client server, which you must replace by some messaging process, this process can also be easy and generate systems with new interfaces and traditional platforms.
Telerik and DevExpress are both vendors of UI components for different technologies (such as .NET Desktop or various web frameworks).
What exactly are you lookup for?