It seems to only be used this way in this book. But it is just a coined word taken from the noun "lame duck" and turned into a verb.
As a noun, a "lame duck" is literally an injured duck who cannot walk (waddle) or walks poorly because it is hurt and in pain.
When seeing a "lame duck" or any injured small animal in the wild, we normally feel pity or sympathy and want to care for it. We feel a desire to claim the animal, take it into our home, and treat it very well, but not let it go until it heals. Maybe we secretly hope that it learns to trust us and love us and stay as a pet.
This is exactly what Fowles means when he writes to "lameduck" something. It's what the main character is doing by kidnapping the girl Miranda. And Miranda uses it to understand why he is doing this to her because she has done it to others. The hints are in each time Miranda uses the word.
pg.90 - "He keeps me absolutely prisoner. But in everything else I am
mistress. ... The same thing happened when I was lameducking Donald last spring. I began to feel he was mine, ..."
This use shows the meaning of lameducking to be about taking away someone or something's freedom but also providing well for it, and having a feeling of ownership. (Imagine caring for an injured duck.)
pg. 131 - "I shall say I feel differently towards him, that I want to
be his friend and lameduck him in London. It won’t be altogether a lie, I feel a responsibility towards him that I don’t really understand. I so often hate him, I think I ought to forever hate him. Yet I don’t always. My pity wins, and I do want to help him."
This time it shows the meaning of lameducking to be about "a feeling of responsibility" and "pity" and a "wanting to help". (Again, imagine caring for an injured duck.)
pg. 139 - "If I use violence I descend to his level. It means that I have no real
belief in the power of reason, and sympathy and humanity. That I lameduck people only because it flatters me, not because I believe they need my sympathy."
This shows that she thinks of lameducking as giving sympathy and thinks that she does it sincerely, not selfishly, because others need her sympathy.
pg. 139 - "Sally Margison. I lameducked her just to show the Vestal Virgins that I was cleverer than they. That I could get her to do things for me that she wouldn’t do for them."
This shows the power of lameducking when the person you are caring for and being sympathetic to does start to like and trust you enough to do what you want. So lameducking can create a power of control over others. (Imagine caring for an injured duck and hoping it becomes your pet, following you and listening to you.)
So lameducking refers to treating other people as if we are caring for an injured animal. Both the positive (sympathy, compassion, caring) and the negative (claiming, controlling, keeping prisoner).