To save time in my workflow, can I work on a .psd and then convert it to a .psb at the end?

@D Fosse nails it. He always does.

For more evidence, refer to the following which was contributed by @davescm some time ago.

It is a treasure, and so is Dave.

You can save your file anytime in a different format. And also a smartobject can be saved in a different format with a simple trick:

Open it and save its content in the new wished format as a copy. Then select the original smartobject in the layers and select "replace content" and navigate to the new file.

This way you can change also smart objects, which were only JPGs (that do not accept layers for saving) to PSD or psb.

Not sure if that is your question, but just in case important to know.

I'm using German Photoshop, so the menu commands could be named differently.

So I think you might be misunderstaninding the power of the Smart Object (.psb) 

Using Smart Objects is GREAT, they are a wonderful addition to Photoshop, I love how they work with filters, and its nice that they give you some freedom in scaling files. However,  its being smart by remembering the current pixel information in an image. Normally if you scale DOWN and image and press ENTER, that's it... you can't scale it back up. But wtih a Smart Object it remembers the pixel information and you can scale it UP AND DOWN. However, you can't scale it BIGGER then its original size and expect it to have the same quality. You are only scaling the Smart Object... the image inside it is the same image, so if you scale it LARGER then the actual image - you will lose quality.  A smart object makes it so if you change your mind, you can scale the image again and not lose quality, or you can update a filter effect, but it can not help you scale an image larger then it actually is. So create your file the actual size that the file will be printed, and make sure your images are given to you large enough that they fit on whatever your final print size will be, and at 300 dpi. The images can be larger then your print size, since you can always scale them down, but with any bitmap, you can't scale it up without loss of quality.

Hope this helps.

mark

I'm sure someone told you that you need to do this at 300 ppi. Which is nonsense.

You don't need more pixels just because it's printed in a large format. Any good file will work for anything, whether it's a magazine spread or a large billboard. You don't need a huge file, you just need a good one. Anywhere between, say, 50-70 ppi will look absolutely spectacular at these sizes. That translates to a file of around 6000-7000 pixels long side, which is what you get from a good camera nowadays.

The bigger it is, the lower the ppi requirement, because it will be seen from farther away. Here's how that works: