Let's assume (dangerous, I know) that the fixed length of your original wire topping lift is just about perfect to let the boom clear your bimini.
So, you cant add a rope 'adjuster' to it without it sagging.
Choice No 1: Reconnect the boom end of the wire topping lift to the back stay and
run a new topping lift ROPE through an available sheave at the mast head from the deck, inside the mast to the boom end.
Choice 2: Cut the existing wire topping lift at the boom end to allow an new adjuster rope
that you will connect to the boom end.
Choice 3: Get a "Boom Kicker" that allows adjustment.
Choice 4: Anybody???