Well, it would be less stressful than leaving them in the snow....
"When Is the latest I could do this and whats the best way to approach it?"
Dig 'em up before it frosts. (Mid October??) Hose 'em down. Trim rootball to size of fist. Trim (butcher) the stems and defoliate them, leaving only a 8-10 inch trunk with nodes. Hose 'em again. Get a sterile pot and sterile growing medium. (If not sterile your aphid friends now have a new winter home.)
Sunny bay window approach:
Put them in a south window, or buy some uber-lights and maintain a growing regimen. New leaf growth will appear in a couple of weeks. Aphids and fungus gnats will appear at some point unless you adhered to sterile cockpit rules above. They might appear anyway.
or--the dungeon approach:
Put your sad, stubby trunks in 1-qt pots in sterile medium. Water one time to settle the medium and put in a cool place with *some* light. Induce a semi-dormancy. Do not put in a sunny window until around February. Foliage prior to this is not desirable. In Feb, feed lightly, consider a pot-up--and pray. If they revive, put 'em back in the dirt when warm and toasty outside, around late May...
or--bonchi:
do bonsai plant with the little trunks. Un-bonsai them when spring returns.
Finally: Advice given by anyone from SoCal, Texas or Florida? They only need to park inside a few weeks, if any. You, in the far north, face a 6-month out of ground experience, a different game plan. Expect some causualties....