Thank you for your questions.
It is possible to stimulate certain brain regions to stimulate finger movement. All our movements follow a neural pathway which is how neurons in our brains interact with muscles. With electrical or magnetic stimulation, scientists have already shown they can control finger movements remotely. You will love this TED video where the presenter shows how one can control a stranger’s fingers with a simple electrical device.
However, as a musician myself (I play the flute), I would suggest practicing! As the saying goes, practice makes perfect =) I used to have problems moving my fingers fast enough for some songs but with diligence, I could overcome the problem.
Many individuals who are paralyzed have unfortunate severed neural pathways that affect the communications between their neurons and muscles. This is why they have difficult exercising voluntary movements. This is not the case for you and me! For our case, it is probably because we have not actively train our brain to use that particular finger and so there are insufficient connections between the neurons and that particular muscle set. Fortunately, our brain is rather plastic which means that it can learn and unlearn things by forming/breaking synaptic connections. Therefore, with sufficient training, you might be able to enhance the number of neuronal synapses devoted for muscle dexterity at your fingers. You might also be keen to check this article out.
I see that there are some websites advising people how to improve their finger dexterity. Why don’t you try and let me know?