Is it possible to build .a or .so library files to use with build app body?
If it is not, is it possible to build an Xcode project with multiple sub-projects?
Sub-projects are supported; they will actually internally build with the build system as static libraries (.a).
However note: that this is slightly different than iOS build procedures in that the build system will automatically treat these libraries as if they had -ObjC -all_load linker flags.