Actually, if you look further down the list (in the menu instead of the toolbar) you will see the Open Drawing command. It is not in your picture you posted because you cropped it above that option. There is no ability to move commands from the context menu to the context toolbar. However, this is an inconsistency between assemblies and parts that really shouldn't be there and is logged as part of SPR 796352. You can vote on that SPR by searching for it in the enhancement request system on the customer portal or contacting your reseller and asking them to put in a vote for you.
This link may take you to the request:
Note that the description doesn't match the exact problem you mention, but it is part of that request in the details.
I knew about the context menu. I was looking for a way to have it in the toolbar.
Thanks for your time and help.
And of course for the link too.
I will vote as soon as I'm back to work.