There is no really good way to do this right now.
If you only have one .asm file, then you can get it assembled using custom build commands from the project manager's context menu. This is quite painful if you have a lot of files, though, as you have to add the custom build target for every file.
Another option is using a makefile, and yet another is to run a batch file from "Tools". Neither of these is really a good solution, though.
