Yes, you can get the FINAL SELLO. Tell the Security Guard that you simply want "SOLO SELLO, Por Favor."
You will be directed or escorted to a staff person who will affix one of the two different sellos to your credential to close it out. Once this is done, this particular credential can no longer be used to support obtaining a Compostela. The specific location of the final sello in your credencial does that.
When you return to complete your walking pilgrimage into Santiago, obtain a new credencial to document that pilgrimage. This will document that you walked more than the minimum 100 km into Santiago. It will be enough for you to get a Compostela.
IF YOU ALSO WANT A CERTIFICATE OF DISTANCE, bring this first, earlier credencial. Although it has been closed for Compostela purposes, it can still be used to establish that you started farther down the route to bolster a request for a longer distance on the Certificate of Distance.
Finally, If you KNOW you will return to finish the entire Camino from your current stopping point, DO NOT seek the Cathedral stamp now. When you return to finish, you can / should simply pick up where you left off. But, on this second go, you must walk at least the final 100 km into Santiago.
When you arrive at Santiago, the stamps will establish that you walked the Camino Primitivo with a calendar break at Lugo. THIS IS NORMAL. Some pilgrims take 3 -5 years to finish and produce the same credencial, collecting stamps in order until they arrive at Santiago.
I hope this helps.