Git: send commit to another branch so I can merge back, without switching branches

Git: send commit to another branch so I can merge back, without switching
branches

So I have 2 branches, the main one and the one I'm working on a parallel
release.
A --> B --> C (master)
\
-> E --> F (parallel)
The parallel branch will always merge from master. Always. And modify upon
it.
A --> B --> C --> D --> H (master)
\ \ *merge*
-> E --> F --> G --> J (parallel)
This is easy to do if I switch branches. But, if I'm working on parallel,
can I do this without switching branches? The problem with switching is
that it takes a long time to go back and forth (specially with Unity 3D,
because it keeps rebuilding the library)!
So say I'm on F, while master is still on A. Then I wanted to make few
commits on master B and C then merge them into G. How would I do it,
again, without switching branches?