Do you really mean that you want to start a new application and use a layout that you have already designed for another application? That seems more likely. If so open the Designer from the new application and select "File ==> Open ...". It might be better to first put a copy of the *.bal file in your new apps "Files" folder and use that copy.
If you move your other app from the main activity to a class or module, then you can share those code in an other program. Be aware that you can put and maintain those shared code on a shared location and refer in an other program to that shared code by not copy the code but point to that shared code. In this way you can divide a program into different bite-sized chunks that you can easily and quickly reuse.