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Combine part with step file

I am brand new to onshape, and haven't done any cad work in about 20 years, so please be patient with me. I suspect this is a really simple task.
I have a device cradle in a STEP file. I need to add a block with a pair of counter-bored screw holes to the back of it so I can 3D print the whole assembly and mount it to a bracket. I've created the block, and I just need to merge it with my original part so I can print it.
I cannot figure out how to work with the step file. My only options appear to be be to update it or download it. This seems like a fully featured program, surely there's a way to do it. So, is there a way I can add a step file to an existing document so I can put this all together?
Comments
When you import the STEP file, it will create a tab with the import (the one with the download/update options) as well as part studio (or multiple if the STEP is an assembly), i.e. another tab with the part in it.
From here you can either model the block directly on there, or since it looks like you have already done it in a separate tab, you can use the derive feature (an transform tool if needed) to bring them in a single part studio and Boolean together before exporting.