One of the American programs to sustain commercial ships available to the American military during war is the Maritime Security Program (MSP):
The MSP provides supplementary funding, currently $5 million per ship annually, for a specific number of U.S.-flagged, privately-owned ships which conduct international commerce but which are available under agreement to respond to Department of Defense (DoD) requirements during war and national emergencies. Currently, the MSP fleet consists of 60 vessels in a mix of container ships, roll-on/roll-off carriers, and heavy lift platforms.
I would like to see a force pool of civilian US-flagged container ships sustained by a program to equip modularized auxiliary cruisers from that pool during both peacetime and war.