Mate the sketch plane of the hole in part B with a plane of part A that you want the hole to be on.
Make the sketch visible.
Click on part a then edit part, click on the plane you want to sketch on then highlight the sketch entity in part B.
Click on convert entities to copy the sketch into part A to make your hole.
When you rebuild this assembly the hole in part A will change as you alter the sketch in part B.
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in assembly A, select part A and invoke the edit part command (it is in the command manager, the pop-up in-context toolbar, and in the right click menu). part B should now be transparent and you can add features to part A, referencing part B or anything else in the assembly.