Fabian,
Let me strongly caution you that the use of a shoulder stock with a 6 inch Commercial Luger would violate the National Firearms Act of 1934 and be a federal felony UNLESS before you own ALL of the required items you file the necessary forms and pay the $200 tax for "making" a short barreled rifle... or SBR.
Authentic Navy Lugers have been exempted from the NFA registration requirements and tax because they have been classified as NFA Curios and Relics... Regular C&R guns do not qualify...
In other words, You can do what you want, but in reality you must get ATFE approval it will cost you $200 more than you have budgeted, and the gun will forever more require that the NFA transfer tax $200 be paid every time it changes ownership...
Whatever you decide... please keep us up to date in photos
Just thought you should know...