In my personal canon, Force-sensitivity is basically a result of how the individual genes of a living individual come together and interact to form that individual; these individuals don't have "Force-sensitive" genes as such -- the way their genetic makeup is arranged simply affects how they're tuned into the Force.

Now, normally this is something one is born with, but there are ways Force-sensitivity can be induced in non-sensitives.

GENETIC MODIFICATION

Altering an individual's genetic makeup can also affect how said individual is tuned into the Force. It doesn't even have to be a major alteration; something as mundane as changing their eye or hair colour could do the job.

As each person is tuned into the Force in a different way, however, it's impossible to figure how each individual should be modified to activate Force-sensitivity within them, so using genetic modification is a largely fruitless method to get the job done.

FORCE ALCHEMY

This is using Force-based sorcery -- light, dark, or otherwise -- to graft Force-sensitivity onto non-sensitives.

GESTATE NON-SENSITIVE & FORCE-SENSITIVE EMBRYOS TOGETHER

Implant a non-sensitive embryo in the same womb (natural or artificial) a Force-sensitive embryo is occupying, and given enough time, Force-sensitivity will "rub off" onto the non-sensitive embryo, bestowing them with the ability to touch the Force.