Normally a cell that has failed in this way will have a markedly lower voltage than the rest, so measurement with a meter usually shows that up.
Adding a new cell to an existing chain in series will often lead to failure, and the older the existing cells, the more likely that is. NiMh battery manufacturers take care to only include cells from one manufacturing batch in each pack, and the best manufacturers/suppliers always select the cells by matching before assembly. The major US supplier Batteryspace came unstuck in this way by failing to do that and getting a bad name for failures. They now do select for matching and their prices have shot up as a result.
So what you're proposing is not recommended.
I have added cells to existing packs in some limited circumstances with both success and failure, so if you do still try this, here's some tips.
Estimate the capacity of the existing cells by calculation from their age. At about three years old, well looked after cells will have around half the original capacity as a guide. Then use a cell of the same estimated capacity.
Before adding the cell, precharge it to the same measured voltage level.
Then make the first pack charge a short part charge, and use the pack for a short while only, never to empty.
On the second charge/discharge, extend those a bit further.
Then use the pack normally. By following that procedure throughout you stand the best chance of bringing the cells all up to a matching balanced state. It's imbalance that can result in reverse charging of one cell by the others, with sometimes disastrous results.
P.S. Alternatively you could just use the 19 cells as an extra pack to extend range, using wired connections. When new and freshly charged, a 24 volt pack will reach around 29 volts, and when discharged, 24 volts. Your 19 cells fully charged will reach, depending on age, around 27.5 volts, so still able to drive the Lafree for some distance before voltage drops to cutout level.
One user who has added a range extending battery in a similar way using a Powacycle pack is pedelecs member "Grandad", and his mod is described
here.
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