If you want to run git submodule update in your conanfile.py, you can simple add a self.run() command after the git clone command:
self.run("git clone https://github.com/conan-io/hello.git")
self.run("cd hello && git submodule update --init --recursive")
Full example:
self.run("git clone https://github.com/seznam/elasticlient.git -b version-0.2")
self.run("cd elasticlient && git submodule update --init --recursive")
Note that you need to cd <directory> && git submodule update