It's real easy for a battery shop to tab 2 new nicads together for you at quite reasonable prices. Interstate Batteries in Arvada has done this for me a couple of times for an ancient DeWalt drill I have.
The problem with soldering is that it is real easy to apply too much heat and break the internal connection to the terminal in the battery. The standard practice is to spot weld those tabs on with a real quick ZOT at several very small spots so that the heat dissipates before it gets into the battery.
I have zapped Nicads many times since I had quite a few portable transmitters, both on the CB bands and the ham bands. What happens (or at least this is the "lore") is that under constant charging or many charge-recharge cycles, "whiskers" grow between the anode and cathode and short out the cells. These whiskers can be blasted out of existence by passing a high current through them.
I used to do this with a standard 12V automotive battery on individual 1.2V Nicads, and it involves eye protection and gloves and it's not a good idea to do it under the hood of the car because of possible gasoline vapor ignition.
On the workbench, a very quick connection is made to the NiCad in the proper charging direction by brushing the connection very quickly, maybe twice. It throws sparks and overdoing it will pop the NiCad. They get fairly warm in this process, and you will never get the same energy capacity as when new, but for extending the useful life for a while, it's OK.
At this point in the brushing and sparking process if you haven't totally ruined the battery, it will show an almost full charge. Let it cool before putting it in the regular charger. This also allows the depolarizing agents to recover somewhat from the zapping.
Sometimes this works well, sometimes it doesn't, but pay attention to what you're doing because throwing sparks around on a workbench can be dangerous, too Use the gloves and the eye protection. I can take no responsibility for results or possible damages. I have no information on any possible similar processes for batteries other than NiCads.
You will have a fire extinguisher nearby, won't you?
Terry, hamming it up since 1974-ish, 230RN